SMTP Error 502: Command Not Implemented

Permanent failure Medium severity Protocol RFC 5321

What it means

SMTP Error 502 means “Command Not Implemented.” The receiving server does not support the SMTP command your server tried to use. This typically occurs when your server attempts to use an SMTP extension like STARTTLS or AUTH that the receiving server has not implemented.

At a glance

Code502

Bounce typeHard (permanent)

SeverityMedium

CategoryProtocol

What to doSuppress the address; do not retry

StandardRFC 5321

What it looks like in your mail logs

502 5.5.1 VRFY command is disabled

What does 502 mean?

SMTP code 502 means the receiving server recognizes the command format but has not implemented that particular command. This is different from 500 (unrecognized command) because the server understands what you are asking for but does not support it.

The most common scenario is attempting STARTTLS with a server that does not support TLS encryption, or using AUTH with a server that does not support SMTP authentication. Some older or minimal mail servers may also return 502 for extended SMTP commands (ESMTP) that they have not implemented.

How 502 plays out

Your server attempts delivery

The recipient server returns a permanent 502 rejection

This is a hard bounce: the message will not be accepted as sent

Suppress the address and fix the root cause before resending

Where 502 sits: soft vs hard bounce

	Soft bounce (4xx)	Hard bounce (5xx)
Nature	Temporary	Permanent
SMTP class	`4xx`	`5xx`
What to do	Let it retry	Suppress the address
Recoverable?	Often	No
502 is		✓ this code

Common causes of 502

Attempting STARTTLS with a server that does not support TLS
Using AUTH command with a server that does not support authentication
Sending ESMTP commands to a server that only supports basic SMTP
Server has deliberately disabled certain commands for security

How to fix 502

Check the server EHLO response to see which extensions are supported
Fall back to basic SMTP commands if extended commands are not supported
If STARTTLS is not supported, consider whether to send without encryption (use opportunistic TLS)
Update your MTA configuration to handle servers with limited capabilities

Frequently asked questions

What does SMTP error 502 "Command not implemented" mean?

SMTP error 502 means the receiving mail server recognized the SMTP command but does not support or has disabled it. This often occurs when a client sends a command that the server's configuration does not allow, such as VRFY, EXPN, or certain authentication methods. The server understands the command syntax but cannot execute it due to administrative restrictions or software limitations.

How do I fix SMTP error 502?

To fix SMTP 502, update your email client settings to match the server's supported commands and authentication methods. Verify the correct SMTP server address, port (587 for TLS, 465 for SSL), and authentication type. If using custom scripts or automated systems, ensure commands are sent in the correct SMTP sequence. Contact your email hosting provider to confirm which SMTP commands and authentication methods their server supports.

Why does my email server return 502 for certain commands?

Mail servers deliberately disable certain SMTP commands for security or administrative reasons. Commands like VRFY (verify address) and EXPN (expand mailing list) are commonly disabled to prevent email address harvesting by spammers. If you receive 502 errors, your client is likely sending a command the server administrator has intentionally turned off. Switch to alternative commands or authentication methods supported by the server.

Related bounce codes

Sources & further reading

RFC 5321 · The standard that defines this reply code RFC 5321, Section 4.2.2 · Defines the 502 reply for a command that is recognized but not implemented

Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes