What does 501 mean?

SMTP 501 indicates parameter syntax errors:

What it means:

The command itself was understood, but the parameters were wrong. Arguments may be malformed, missing, or invalid.

Common causes:

Invalid email address format: Malformed recipient or sender address. Missing required parameters: Command incomplete. Invalid characters: Unsupported characters in parameters. Encoding issues: Parameter encoding problems.

Typical message examples:

"501 Syntax error in parameters" "501 Bad recipient address syntax" "501 Invalid address format"

Response:

Check the specific address or parameter causing the error. Validate email address formats. Review your mail configuration.

501 says "I understand the command, but not the details." The parameters need correction.