A Complete Guide to the SMTP HELO Command

The HELO process is the “Hello, it’s me” of the email world, essential for SMTP servers to greet each other politely before communicating. 👋 Understanding HELO is key to successful mail delivery.

This comprehensive guide demystifies everything about SMTP’s pivotal HELO/EHLO handshake. You’ll learn how HELO works, its core purpose, proper configuration, troubleshooting tips, and more! Time to become a HELO master.

Page Contents

What is the SMTP HELO Command?

The HELO command is one of the basic commands used in the Simple Mail Transfer Protocol (SMTP) to initiate a conversation between an email client and SMTP email server.

HELO Defined

HELO stands for “Hello”. It is the command an SMTP client sends to introduce itself and establish a connection with the receiving SMTP server when attempting to send an email.

The HELO command serves several important purposes:

  • It represents the first step in the SMTP protocol handshake. HELO opens up communication between the sending and receiving SMTP hosts.
  • It identifies the sending client/server to the receiving server. The domain name specified after the HELO command indicates where the email originated from.
  • It verifies that the receiving SMTP server is ready and willing to accept incoming mail transactions from the client.
  • It resets and clears all previous transaction information on the server to prepare for the new email transfer.

So in summary, the core purpose of HELO is to initiate and establish an active SMTP session between the sending and receiving servers to enable email transfers.

HELO Command Structure

The basic syntax for the HELO command is:

HELO sending.server.domain

Where “sending.server.domain” is the fully qualified domain name (FQDN) of the server sending the SMTP email.

For example, if an email is being sent from the client server “mail.example.com”, the HELO command would be:

HELO mail.example.com

Normal Server Response to HELO

When a receiving SMTP server is ready to start a new SMTP transaction, it will reply to a client’s HELO command with response code 250.

For example:

Client: HELO mail.example.com
Server: 250 mail.example.com Hello 

This 250 response indicates the server is ready and allows the client to proceed sending its email.

Difference Between HELO and EHLO

EHLO is an enhanced version of the standard HELO command that supports SMTP protocol extensions. It was introduced with the Extended SMTP (ESMTP) specification.

The key differences between HELO and EHLO include:

  • EHLO returns a multi-line response from the server listing which SMTP service extensions and enhanced features it supports. This allows the client to take advantage of these extensions.
  • HELO returns a single line response without detailing SMTP extensions present on the server.
  • EHLO enables added SMTP functionality like SMTP-AUTH for login, STARTTLS for SSL encryption, 8BITMIME for non-ASCII text, and more.
  • HELO only establishes a basic SMTP connection without extra capabilities.
  • All SMTP servers support HELO as it is part of core SMTP spec.
  • Some older SMTP servers may not support EHLO since it is an ESMTP extension.

So in essence, EHLO does everything HELO does in terms of initiating an SMTP connection, but also enables extended SMTP features if the server supports them.

When to use EHLO instead of HELO

Given its wider feature support, EHLO is generally recommended for usage in most client software and scripts implementing SMTP:

  • Try EHLO first to check if the server supports SMTP extensions.
  • If EHLO returns an error, fallback to plain HELO to establish a basic SMTP connection.
  • Use EHLO again after STARTTLS to reinitialize ESMTP capabilities.

So unless dealing with a legacy SMTP system, EHLO is typically preferable over HELO in modern applications.

However, HELO remains relevant as a guaranteed fallback due to its inclusion in the original SMTP RFC spec. All conforming SMTP servers still need to support HELO even if they don’t implement ESMTP extensions.

In short

  • HELO initiates a connection between SMTP client and server to enable email sending.
  • EHLO supports ESMTP extensions like AUTH and STARTTLS when present on the server.
  • EHLO provides enhanced capabilities whereas HELO establishes a basic SMTP session.
  • Try EHLO first, and fallback to HELO if needed for wider compatibility.

So in summary, the SMTP HELO/EHLO command plays a core role in starting SMTP conversations, identifying clients, and indicating server capabilities. Understanding its function and proper use is important for effective email transmission.

How the SMTP HELO Command Works

Now that we’ve defined the SMTP HELO command, let’s look at how it functions during the SMTP process to open up an active email session.

Overview of the SMTP Protocol Handshake

To send an email using SMTP, the sending and receiving servers must go through connection handshake steps:

  1. The SMTP client contacts the receiving SMTP server on port 25 and attempts to establish a TCP connection.
  2. The server confirms it is listening and ready by returning a 220 response code. For example:
S: 220 smtp.example.com ESMTP Ready
  1. The client sends the HELO command to identify itself and initiate the SMTP conversation.
  2. The server responds affirmatively to the HELO with a 250 code:
C: HELO mail.sendingserver.com
S: 250 Hello mail.sendingserver.com
  1. The client and server exchange SMTP commands like MAIL FROM, RCPT TO, DATA, QUIT to transfer the actual email.

This initial “handshake” using the HELO command sets up the active SMTP session that makes transferring email possible.

Sending the HELO Command

The SMTP client sends the HELO command to the server immediately after establishing the TCP connection.

The basic syntax is:

HELO sending.server.domain

For example, a mail server named “mail.example.com” would send:

HELO mail.example.com

Best Practices When Sending HELO:

  • Use your server’s real fully qualified domain name.
  • Avoid fake or generic IDs like “localhost” or “mailserver1”.
  • If possible, verify the returning HELO IP address matches the server’s A record.
  • Some servers may reject the handshake if HELO is invalid or does not match the connecting IP.

SMTP Server Responses to HELO

Based on RFC 5321, a server should reply to HELO with one of the following response codes:

  • 250 : Requested mail action okay, completed. This indicates the server accepts the HELO and is ready to proceed.
  • 500 : Syntax error, command unrecognized. This means the command was rejected or formatted improperly.
  • 501 : Syntax error in parameters or arguments. Similar to 500 but pertains to the arguments given.
  • 504 : Command parameter not implemented. The server does not support the requested HELO parameter.
  • 421 : Service not available, closing transmission channel. This indicates the server is shutting down the SMTP connection.

How the SMTP Server Processes HELO

When the SMTP server receives a valid HELO command, here is what generally happens:

  1. The server parses the HELO and separates the command word from the domain argument.
  2. It resets any previous SMTP session state to prepare for new transaction.
  3. It compares the HELO domain to a whitelist or blacklist based on policy.
  4. Assuming not blocked, it stores the client domain for validation and debugging.
  5. The server formulates a 250 response with the client domain.
  6. It sends the 250 reply code to confirm acceptance of the HELO.

Once the client receives the 250 OK from the server, the SMTP connection is established and ready for further commands to send mail.

How HELO Fits Into the SMTP Transaction Process

The HELO command is the opening sequence in the larger SMTP process:

  1. Open connection – Client contacts server on port 25
  2. HELO – Client identifies itself to server
  3. MAIL FROM – Specifies sender’s email address
  4. RCPT TO – Specifies recipient’s email address
  5. DATA – Sends contents of the email
  6. QUIT – Closes the connection

HELO initiates this transaction and opens up the connection. Further SMTP commands rely on that active session started by HELO.

HELO Establishes an Interactive SMTP Connection

In summary, the core function of HELO is to open up an interactive SMTP session between the sending and receiving servers. This enables the servers to communicate and transmit email messages.

Key points about how HELO accomplishes this:

  • It provides a formal greeting and introduction from client to server.
  • It gives the server a verified identity and return address for the client.
  • It clears any previous session state and resets the connection.
  • It confirms that the receiving server is listening and ready.
  • It allows validation and checking of the client domain.
  • It initiates the back-and-forth SMTP command sequencing.

Without a valid HELO command starting the process, no email can be sent down the channel. So HELO plays a crucial role in getting the SMTP session up and running.

Example HELO Conversation

Here is an example showing the typical HELO exchange:

(Connection Established)

C: HELO mail.sendingserver.com
S: 250 Hello mail.sendingserver.com, pleased to meet you

C: MAIL FROM: [email protected]
S: 250 OK

This simple HELO handshake opens up the communication so the client and server can then proceed with the email transfer.

In nutshell

  • HELO is essential for initiating an interactive SMTP session.
  • It resets server state and confirms the server is listening.
  • Valid HELO identifies the client and enables further SMTP commands.
  • Servers reply to HELO with 250 OK or an error code.
  • HELO must complete successfully for email sending to occur.

Understanding the mechanics of this core SMTP command is key to robust email delivery.

Key Uses and Functions of the SMTP HELO Command

Now that we’ve explored what the HELO command is and how it works during SMTP sessions, let’s look at some of its major uses and functions that make it an indispensable part of the email sending process.

Initiates the SMTP Conversation

The primary purpose of the HELO command is to initiate an interactive SMTP conversation between two servers.

When a client wants to deliver an email, it must first introduce itself and open up a dialog with the receiving SMTP system. HELO does this by kicking off the protocol handshake.

Some key ways HELO starts the SMTP dialog:

  • It represents the first command issued by the client after establishing a TCP connection.
  • The server responds to HELO affirmatively before any other SMTP commands can be issued.
  • It signals both systems are present and ready to proceed with the transaction.
  • HELO resets previous session data, preparing for a new dialog.
  • It tells the receiving host that the sender wants to start communicating.

Without the opening HELO command, no further SMTP conversation can happen between the two systems.

Introduces the Sending Server

A core function of HELO is introducing the sending client system to the receiving SMTP server.

The domain name passed with the HELO command identifies where the email originated from. This serves several purposes:

  • Puts the receiving host on notice regarding the sending domain.
  • Associates the connecting client with a verified domain identity.
  • Allows the receiving server to log and track the source.
  • Enables validation that the domain is legitimate and authorized.
  • Provides a reverse-path for diagnosing issues and troubleshooting.

So HELO gives the receiving host key information about the sending client necessary to enable the transaction.

Opens Up an SMTP Connection

We’ve established that HELO initiates a dialog between mailing systems. But on a technical level, its key function is opening up an active SMTP connection and session between the client and server.

Several things happen behind the scenes when the servers exchange HELO:

  • The server process accepts the new TCP connection from client.
  • It spawns a new dedicated SMTP service thread to handle the session.
  • The thread state is initialized and reset to receive the new HELO.
  • Resources like memory, buffers, and disk space are allocated to the thread.
  • Session logging and tracking is started.
  • The service thread sends a 220 greeting when ready.

So HELO causes the server to spin up and initialize an SMTP connection ready to handle subsequent commands.

Enables Sending of Email

The end goal of the SMTP process is transferring email messages reliably from senders to recipients. The HELO command starts the handshake required to achieve this goal.

Some ways HELO sets up the email transfer:

  • It establishes the active session for sending data.
  • It introduces the sender with valid identity for authentication.
  • It allows negotiation of extensions like encryption.
  • It resets the server so email has a clear path.
  • It designates the sender has mail to transmit to this server.
  • It opens the connection for SMTP traffic to flow through.

So in essence, HELO enables the client to send email on a technical level by starting the SMTP service handshake.

Additional HELO Functions

Beyond the major uses above, the HELO command also serves some additional functional purposes:

  • Provides server software identity in the 220 greeting.
  • Gives an extra protocol step for security screening and policies.
  • Allows implementation of greylisting spam filters using HELO.
  • Serves as input for Botnet and RBL checking using the client IP.
  • Lets administrators customize the SMTP service greeting message.
  • Offers an additional logging and auditing checkpoint.

So HELO has some secondary purposes beyond just initiating the SMTP conversation.

Key Takeaways on HELO Function

  • Starts the dialog between sending and receiving SMTP systems.
  • Introduces and identifies the sending domain to receiver.
  • Triggers opening of dedicated SMTP service connection.
  • Allows server to allocate resources to handle the session.
  • Resets previous state to prepare for new email transfer.
  • Enables progression through subsequent SMTP commands.
  • Sets up the pathway over which email is sent.

Understanding these core functions gives insight into the crucial role of HELO.

HELO vs EHLO: What’s the Difference?

Now that we’ve fully explored the SMTP HELO command, you may be wondering how it compares to the similar EHLO command. While HELO establishes a baseline SMTP connection, EHLO supports SMTP extensions for added functionality. In this section, we’ll look at what makes EHLO different and when to use it instead of plain old HELO.

Definition of Extended SMTP (ESMTP)

EHLO stands for “Extended HELO” and is defined in the Extended Simple Mail Transfer Protocol (ESMTP) specification, RFC 1869.

ESMTP builds on the original SMTP standard by introducing various enhancements and extensions such as:

  • SMTP Authentication for login security
  • Transport Layer Security (TLS) for encrypted connections
  • 8BITMIME for transmitting 8-bit data
  • Internationalized email addresses
  • Delivery status notifications
  • Message size declarations
  • Pipelining for improved transmission efficiency

So ESMTP offers a variety of advanced capabilities beyond vanilla SMTP. And the EHLO command is the way clients can use these extended features.

EHLO Supports SMTP Extensions Whereas HELO Does Not

The core difference between HELO and EHLO is that EHLO enables the use of ESMTP extensions if the server supports them.

The classic HELO command establishes a baseline SMTP connection. The client and server can only use functionality defined in the original SMTP RFCs.

In contrast, EHLO negotiates an upgraded SMTP session with extended capabilities. The client and server can take advantage of advanced ESMTP features to enhance the mail transmission.

So if you need added SMTP functionality, EHLO is required to activate it whereas HELO only gives you plain SMTP.

Server Features Advertised in EHLO Response

One key way EHLO exposes extended SMTP capabilities is through the multi-line response it returns.

After the client issues an EHLO command, the server sends back a reply listing the supported ESMTP extensions:

S: 250-mail.server.com Hello client.com
S: 250-SIZE 10240000
S: 250-ETRN

This allows the SMTP client to tailor its behavior and use these advertised features for the remainder of the connection.

In contrast, HELO just returns a basic single-line response.

When to Use EHLO Instead of HELO

Given its extension support, when should you use EHLO versus HELO?

In general:

  • Always try EHLO first – This lets you detect server extensions.
  • Fallback to HELO if EHLO returns an error – Indicates an old SMTP server.
  • Use EHLO again after STARTTLS to reinitialize encryption capabilities.
  • Use HELO if needing generic SMTP without extensions.

So unless dealing with a legacy server, EHLO is typically preferable in modern applications. But HELO is still relevant as a guaranteed fallback option.

SMTP Extensions Enabled by EHLO

Now let’s look at some of the common ESMTP extensions that EHLO enables if supported by server:

STARTTLS – Allows encrypted TLS connections for secure SMTP sessions.

8BITMIME – Permits sending 8-bit data like UTF-8, expanding beyond 7-bit ASCII.

SMTPUTF8 – Enables UTF8 support for internationalized email addresses.

SIZE – Client and server can declare maximum message size to accept.

PIPELINING – Optimizes transmission by allowing concurrent commands.

DSN – Enables delivery status notification for email tracking.

ETRN – Permits triggering mail queue processing on demand.

AUTH – Allows SMTP authentication mechanisms like LOGIN and PLAIN.

So EHLO serves as the gateway for unlocking many powerful SMTP extensions if supported.

EHLO May Be Required for Modern Email Functions

Given the capabilities it enables, EHLO is often required to perform certain modern mail functions:

  • Encrypted connections using STARTTLS need EHLO.
  • SMTP authentication using AUTH relies on EHLO.
  • 8-bit encoded data like attachments require EHLO.
  • Better Unicode support is enabled by SMTPUTF8 under EHLO.
  • Delivery confirmations via DSN depend on EHLO.
  • Larger modern emails benefit from SIZE extensions in EHLO.

This means HELO alone may be insufficient in many real-world email scenarios today.

EHLO Drawbacks

However, EHLO does have some potential drawbacks and pitfalls to watch out for:

  • Not universally supported on older SMTP servers – Fallback to HELO required.
  • Extended features can open additional attack surface if poorly implemented.
  • More opportunity for bugs due to added protocol complexity.
  • Specific extensions may be blocked by antispam policies.
  • Requires ESMTP logic in clients vs universal HELO support.

So while EHLO enables advanced capabilities, plain HELO may be more robust and foolproof in some situations.

Key Takeaways on EHLO

  • Supports Extended SMTP (ESMTP) capabilities if server allows it.
  • Allows use of extensions like STARTTLS, AUTH, 8BITMIME.
  • Advertises supported features in multi-line response.
  • Preferable for modern mail usage in most cases.
  • Can fallback to HELO for wider compatibility if needed.
  • Offers advantages but also some potential drawbacks.
  • May be required for encryption, authentication, attachments.

Understanding EHLO helps leverage SMTP extensions when available.

Importance of a Valid SMTP HELO Identification

We’ve covered the mechanics of the HELO command. Now let’s discuss the importance of configuring a valid HELO identifier for security, deliverability, and compliance with SMTP standards.

HELO Domain Validation by Receiving Server

A core reason to properly identify your domain in the HELO command is that the receiving server may validate it to filter out suspicious and fraudulent activity.

Here are some ways servers commonly check and confirm HELO domains:

  • Reverse DNS lookup – Validates the sending IP matches the HELO domain.
  • Domain resolution check – Confirms the HELO domain exists in DNS.
  • Real-time blacklists (RBLs) – Blocks IP addresses linked to spamming.
  • Domain reputation checks – Detects HELO domains associated with abuse.
  • Allow/blocklists – Accepts or rejects specific HELO names.
  • DomainKeys Identified Mail (DKIM) – Authenticates domain ownership.
  • Sender Policy Framework (SPF) – Permits IP addresses designated in DNS.

Failing these validations may lead to HELO rejection, degraded deliverability, or flagging as spam.

Preventing Invalid or Suspicious HELO IDs

Some common HELO configurations that can appear questionable or invalid to servers include:

  • Placeholder domains like “mailserver1”.
  • Localhost or local network names.
  • IP addresses instead of domains.
  • Generic IDs such as “outbound.mail.server”.
  • Unresolved or fake HELO names with no DNS record.
  • Domains not owned by the sending organization.

HELO best practice is to use your true sending domain name matching the server IP.

Benefits of Proper HELO Configuration

Setting a valid HELO identifier provides several deliverability and compliance benefits:

  • Increased inboxing rates from passing receiving server checks.
  • Improved sender reputation by using real organization domains.
  • Compliance with standards like RFC 5321.
  • Prevention of domain name spoofing.
  • Safer vendor email sourcing by identifying partners.
  • More useful diagnostics from valid reverse-path.
  • Reduced spam flagging when HELO aligns with sending infrastructure.

So taking the time to properly identify your mail servers is worthwhile.

Best Practices for SMTP HELO Configuration

To maximize deliverability and avoid issues, here are some best practices for configuring HELO identifiers:

  • Use a real public domain owned by your organization if possible.
  • Make HELO fully qualified such as “mail.company.com”.
  • Match theresolvername to your Server’s A record.
  • Set up proper rDNS and PTR records for HELO IP addresses.
  • Avoid generic names like mailserver1, mail, SMTP01, etc.
  • Never use localhost, loopback IPs, private subnets.
  • Consider configuring unique HELO IDs per sender to isolate deliverability.
  • Change HELO if sending from a new IP address.

Following these simple guidelines helps ensure your HELO passes validation checks and achieves optimal deliverability.

In short

  • Receiving servers often validate the HELO domain and rejecting invalid ones.
  • Use real public domain names matching server IPs for deliverability.
  • Avoid fake, generic, localhost HELO identifiers.
  • Proper rDNS and PTR records are important for trust.
  • Well-configured HELO improves inboxing rates and sender reputation.

So put some thought into properly identifying your outbound mail servers in the HELO handshake. Doing so helps email flow smoothly to its destination.

Common SMTP HELO Error Codes and Troubleshooting

Even with proper configuration, sometimes the SMTP HELO command can fail with unexpected errors. In this section, we’ll look at some common HELO error codes and how to troubleshoot issues.

5XX Errors – Invalid HELO or Unauthenticated

The 5XX class of response codes indicates the SMTP server has rejected the HELO command or client authentication is required first:

  • 500 – Syntax error, unrecognized command.
  • 501 – Syntax error in parameters or arguments.
  • 504 – Command parameter not implemented.
  • 530 – Authentication required before HELO accepted.
  • 550 – Requested action not taken: HELO invalid or relay access denied.

For 5XX errors, first ensure HELO syntax is valid and re-try the handshake. If that fails, authentication or TLS may be needed if they are required by the receiving server.

4XX Errors – Server Cannot Process Command

4XX response codes indicate a temporary issue on the server side:

  • 421 – Service not available, closing transmission channel.
  • 450 – Requested mail action not taken: mailbox unavailable.
  • 451 – Local error in processing request.
  • 452 – Insufficient system storage for processing HELO.

The server is indicating it has a problem or resource limitation preventing HELO processing. This may be a temporary issue resolvable by waiting and retrying the HELO handshake later.

Fixing HELO Issues to Improve Deliverability

If you are encountering frequent HELO errors, here are some troubleshooting steps:

  • Check HELO syntax – Confirm command is valid format such as HELO mail.domain.com.
  • Verify DNS resolution – Ensure HELO domain resolves and has proper rDNS.
  • Change HELO domain – Attempt different valid server domain names.
  • Try EHLO – See if Extended SMTP works better than basic HELO.
  • Consult server logs – Look for clues in SMTP server error logs.
  • Consider authentication – Try enabling SMTP AUTH if supported.
  • Enable TLS encryption – Use STARTTLS to secure connection if required.
  • Check reputation – Use blacklist tools to detect problems with IP or domain.
  • Whitelist server – Add client IP to server allowed list if possible.

With some analysis and trial & error, the specific cause of HELO rejection can usually be identified and fixed.

Debugging Invalid HELO Name Error 550

A common HELO error is the 550 “Invalid HELO name” response:

C: HELO invalid.domain.com
S: 550 Invalid HELO name

This indicates the domain provided is not valid or permitted for HELO. Some potential causes include:

  • Unresolved domain – No DNS entry exists for the given HELO.
  • HELO domain mismatch – Domain does not match client IP address.
  • Blocked IP reputation – Server rejects traffic from the client IP.
  • Domain blacklisted – The HELO domain itself is seen as abusive.
  • Invalid syntax – HELO is malformed or does not include domain.
  • Relay access denied – Server policy blocks relay from this HELO.

Solving “Invalid HELO Name” Errors

Here are some steps to resolve the common 550 error:

  • Double check HELO domain validity – Test DNS resolution.
  • Verify PTR and rDNS records match – Use nslookup to check.
  • Try different sending server IP – See if problem is tied to specific IP reputation.
  • Analyze SMTP server logs for additional clues.
  • Change HELO identity – Attempt valid alternate domain names.
  • Consider SPF and DKIM – Ensure HELO aligns with sender identity.
  • Check blacklists – Confirm server IP and HELO domain are not listed.
  • Request whitelisting – Ask receiving domain to add client IP to allowed list.
  • Enable TLS and authenticate – May be required before allowing relay.

With debugging and analysis, the root cause of 550 errors can typically be uncovered and mitigated.

Tools to Validate SMTP HELO DNS Setup

Here are some useful tools available to validate DNS settings related to HELO:

  • Telnet – Manual SMTP debugging connects to test HELO.
  • nslookup – Queries DNS for HELO IP address and rDNS.
  • MXToolbox Blacklist Check – Checks HELO domain and IP against RBLs.
  • Mail-Tester – Validates DNS setup for domains.
  • mxtoolbox HELO Check – Specifically analyzes SMTP HELO configuration.
  • DNSViz – Visualizes DNS server configuration and validation.

Using these tools can help identify and resolve invalid HELO configurations triggering 550 errors.

Key Takeaways on HELO Troubleshooting

  • Watch for response codes like 500, 501, 504, 530 indicating rejection.
  • Double check HELO syntax, domain validity, DNS, rDNS, and IP reputation.
  • Common 550 error means the HELO identity is considered invalid.
  • Try different domains, enable TLS, or request whitelisting to resolve.
  • Use SMTP debug tools to analyze HELO acceptance in detail.

With some targeted troubleshooting, most HELO issues can be corrected to enable reliable email delivery.

Using the SMTP HELO Command in Email Clients and Servers

Now that we understand the ins and outs of HELO, let’s look at how it can be sent manually via Telnet and examine support in common email clients and servers.

Sending HELO Manually Using Telnet

Because SMTP is a text based protocol, you can interact with SMTP servers manually using the Telnet tool.

To send HELO yourself with Telnet:

  1. Open Terminal or Command Prompt and enter telnet smtp.example.com 25 to connect to the server on port 25.
  2. If the connection succeeds, the server will display its hostname and SMTP greeting ending in response code 220.
  3. Enter the HELO command like HELO myclient.com.
  4. The server will hopefully reply with 250 Hello myclient.com indicating HELO was accepted.

You can then proceed to send additional SMTP commands like MAIL FROM and RCPT TO to continue a manual SMTP conversation.

Telnet is handy for testing core SMTP functionality like HELO when debugging issues.

HELO Support in Email Clients

All major email clients that support SMTP naturally implement the HELO command under the hood during mail sending:

  • Microsoft Outlook – Uses SMTP HELO when sending through Office 365 or other SMTP servers.
  • Apple Mail – macOS Mail app sends HELO when delivering messages.
  • Mozilla Thunderbird – Open source Thunderbird issues HELO to mail servers.
  • Gmail – The Gmail web client implicitly utilizes HELO to deliver your messages.
  • Yahoo Mail – Yahoo’s webmail implicitly includes HELO when sending your mail.

So while hidden behind graphical interfaces, all these email clients rely on the SMTP HELO command to enable sending.

SMTP Server Support for HELO and EHLO

All conforming SMTP server platforms implement support for the required HELO command:

  • Microsoft Exchange – Exchange servers support HELO and the enhanced EHLO.
  • Sendmail – The popular open source MTA accepts HELO and EHLO.
  • Postfix – This fast MTA for Linux uses HELO plus extensions from EHLO.
  • qmail – The secure qmail MTA issues a customized greeting after HELO.
  • Exim – The Exim mail server for Unix-like systems handles HELO and EHLO.
  • Oracle Communications Messaging Server – Formerly Sun Java System Messaging Server accepts HELO and EHLO.

The various SMTP servers may offer configurable options around HELO but at minimum will accept valid commands that align with RFC specifications.

Testing HELO Behavior of Email Servers

To see HELO handling first-hand on your mail servers, useful debugging tools include:

  • Telnet – Manually connect and send SMTP commands including HELO.
  • swaks – Command line SMTP client that displays server responses.
  • nc – Netcat can pass raw SMTP traffic with customizable HELO.
  • SMTP Reply Code Tester – Online tool testing HELO response codes.

So both email clients and servers should all have HELO implemented given it is foundational to SMTP, but debugging tools allow further inspection as needed.

In nutshell

  • HELO can be manually sent using Telnet for testing.
  • Email clients implicitly use HELO when sending mail behind the scenes.
  • All standard SMTP servers support HELO and generally EHLO too.
  • Test tools like swaks help analyze HELO behavior in depth.

Gaining visibility into HELO functionality in clients and servers helps troubleshoot issues.

SMTP HELO Command in Programming and Scripts

For developers working in various languages, let’s take a look at how to use the SMTP HELO command in code when interacting with mail servers.

Sending HELO in Python

Python’s standard library includes SMTP support for sending email. HELO is implemented in the smtplib module.

A simple example initiating an SMTP connection and issuing HELO in Python:

import smtplib

server = smtplib.SMTP('smtp.example.com', 25)
server.ehlo() ## Or server.helo() 

The smtplib module handles creating the TCP socket, establishing the SMTP session, and sending the EHLO/HELO command automatically when .ehlo() or .helo() is invoked.


In .NET, the System.Net.Mail.SmtpClient class is used to connect to SMTP servers.

By default, SmtpClient will send the EHLO command rather than HELO:

var client = new SmtpClient("smtp.example.com");
client.Send("[email protected]", "[email protected]", "Subject", "Body"); 

Behind the scenes, SmtpClient will establish the TCP socket, EHLO, MAIL FROM, RCPT TO, and DATA without any additional code.

There is no way to force SmtpClient to use HELO instead of EHLO if the server supports ESMTP.

HELO in Java Mail SMTP

Java’s javax.mail package provides SMTP capability. The Transport class handles connecting to the mail server and issuing SMTP commands as needed:

Transport transport = session.getTransport("smtp");
transport.connect("smtp.example.com", 25, "user", "password");
transport.sendMessage(message, message.getAllRecipients());  

Like .NET’s SmtpClient, Java Mail’s Transport will send EHLO automatically when connecting and managing the SMTP conversation.

Other Languages and SMTP Clients

Most programming languages aimed at web development provide SMTP client libraries and HELO support:

  • Ruby – Ruby’s Net::SMTP module connects and sends HELO.
  • PHP – The SMTP class handles SMTP connections and HELO.
  • Node.js – NPM packages like Nodemailer take care of HELO.
  • Perl – The Net::SMTP Perl module abstracts HELO details.
  • Go – Packages like MailGo simplify SMTP sending including HELO.
  • Rust – Crates like Letterbox provide an SMTP client with HELO.

So HELO capability is widely available across languages when leveraging the appropriate mail libraries.

Implementing an SMTP Server Supporting HELO

On the server side, many frameworks exist for building custom SMTP services. These will need to be configured to properly implement the required HELO command.

Some options include:

  • Pythonaiosmtpd is an asyncio based SMTP server library.
  • Node.jssimplesmtp provides a basic SMTP server.
  • C# – Can build a custom SMTP server with Socket class.
  • Javasimple-java-mail-server is an open source library option.
  • Ruby – Gems like ruby-smtp-server available.

Regardless of language chosen, be sure to follow RFC guidelines when implementing the required HELO command and response.

In short

  • SMTP client libraries simplify HELO usage in most languages.
  • C# and Java SMTP classes only send EHLO automatically.
  • Building a custom SMTP server requires manually implementing HELO logic.
  • RFC specs should be followed closely when coding HELO behavior.

Understanding HELO capabilities in your language of choice helps building mail applications.

The Future of the SMTP HELO Command

The core SMTP HELO command has remained largely stable since the initial SMTP RFCs were published. But looking ahead, what does the future potentially hold for HELO?

Current RFC Standards for HELO

The HELO command was first defined in the original 1982 SMTP RFC 821. It has since been clarified and refined through the years:

  • RFC 821 (1982) – Initially defined SMTP protocol including HELO.
  • RFC 1123 (1989) – Further mandated and standardized HELO.
  • RFC 5321 (2008) – Tightened and expanded HELO and SMTP specification.

So the current HELO command we’ve covered is formally defined in RFC 5321 from 2008.

This RFC clarifies and expands on the role of HELO within the SMTP process. Overall though, HELO’s function has remained largely stable since inception.

Potential evolutions of HELO in newer SMTP standards

Looking ahead, how might HELO change or evolve in future versions of SMTP? Some possibilities include:

  • Additional validation requirements – For example, requiring TLS or stricter domain checks.
  • Stronger identity mechanisms – Moving beyond HELO domains to identifiers verifiable with DKIM, DMARC, SPF etc.
  • Expanded information exchange – HELO providing more upfront insight into client capabilities.
  • Integration with email authentication – For example, presence of SMTP AUTH could impact HELO behavior.
  • Support for encrypted connections – Potentially integrating with TLS to provide certificate info.
  • Role in spam and abuse prevention – Evolving HELO policies aimed at mitigating threats.
  • Formalizing error codes – More codified guidance on proper error responses.
  • Backwards compatibility considerations – Balancing innovation with legacy server support.

The HELO-EHLO Distinction Could Blur

Another possibility is HELO and EHLO merging into a single concept in future SMTP releases.

With EHLO’s functional superset, the case for a separate HELO command may diminish over time if ESMTP capabilities become mandatory.

But significant legacy server support implications could discourage pursuit of this path.

In short

  • The core function of HELO has proven durable over decades to date.
  • But opportunities exist for modernization and security improvements.
  • Changes would need to maintain backward compatibility.
  • HELO could eventually be absorbed into EHLO in the long term.
  • Any evolution would require widespread consensus and adoption.

While radical change is unlikely given its pivotal role, it will be interesting to see if HELO is refined in future SMTP versions as needs arise.

Key Takeaways on the SMTP HELO Command

Now that we’ve thoroughly explored the HELO command and its usage, let’s review some of the key takeaways:

  • HELO initiates the SMTP protocol handshake between email servers, kicking off the conversation.
  • It identifies the sending client to the receiving SMTP system.
  • The client domain in HELO can be validated to improve security and prevent abuse.
  • EHLO enables ESMTP extensions like STARTTLS, AUTH, and others. HELO only establishes a baseline SMTP connection.
  • HELO acceptance is required for the email transaction process to begin. Rejection blocks mail sending.
  • Proper HELO configuration using real domain names is important for deliverability.
  • Common HELO errors include 550 invalid name and 5XX authentication required.
  • Email clients and servers rely on HELO even if it runs behind the scenes.
  • SMTP libraries simplify HELO implementation in code for most programming languages.
  • The core HELO command has remained stable for decades but evolutions are possible.

So in summary, HELO plays a foundational role in SMTP mail delivery by initiating the all-important server-to-server conversation. Understanding its purpose, proper configuration, and common errors is key to email success.

Frequently Asked Questions About the SMTP HELO Command

Let’s wrap up with some common FAQs on the ins and outs of HELO.

Q: What does the SMTP HELO command do?

HELO initiates the SMTP protocol handshake between email servers. It identifies the sending client and enables the receiving server to validate the connection.

Q: Is HELO required for sending email?

Yes, a successful HELO handshake is mandatory for SMTP mail transmission to occur. The server must accept the client’s HELO command before further mail actions can proceed.

Q: What is the difference between HELO and EHLO?

EHLO supports Extended SMTP (ESMTP) capabilities like authentication and encryption. HELO establishes only a basic SMTP connection. EHLO is preferable unless the receiving server is too old to support extensions.

Q: What should I set as my HELO domain?

Use a real, fully qualified domain name matching the sending server’s IP address. Avoid fake or generic names that could fail validation and hurt deliverability.

Q: I’m getting a 550 Invalid HELO error. How do I fix it?

Check that the HELO domain resolves in DNS and has proper rDNS set up. Try different valid domains and server IPs. Verify the domain and IP are not blacklisted. Enable TLS/AUTH if required.

Q: How do I send HELO from the command line or in code?

You can manually send HELO using Telnet or an SMTP library. Python, C#, Java, and other languages include SMTP client classes that handle HELO when connecting.

Q: Can I force .NET’s SmtpClient to use HELO instead of EHLO?

No. The .NET SmtpClient will always send EHLO automatically. There is no property to change this behavior or force HELO.

Q: What might the future hold for changes to the SMTP HELO command?

Potential evolutions include tighter identity/validation, increased security, better error handling, and absorbing HELO capabilities into EHLO. But major changes are unlikely given the need for backward compatibility.

Q: Where is HELO formally defined?

The current standard is RFC 5321 from 2008. RFC 821 originally defined SMTP including HELO back in 1982.