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.
What Does Error 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.
Common Causes
- 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 Error 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
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.
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.
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.