What does 500 mean?

SMTP 500 indicates a syntax error in the command:

What it means:

The receiving server did not understand the command you sent. The command may be malformed, unsupported, or out of sequence.

Common causes:

Protocol errors: Commands sent incorrectly. Software bugs: Mail client or server issues. Incompatibility: Extended commands not supported. Encoding problems: Character set issues in commands.

Typical message examples:

"500 Syntax error, command unrecognized" "500 Unrecognized command" "500 Error: bad syntax"

Response:

This is typically a sender-side issue. Check your mail server configuration. Review SMTP protocol compliance.

500 says "I don't understand." The problem is in what you sent, not who you sent it to.