SMTP codes
SMTP 501 Syntax error in parameters
The command arguments are wrong: most often a malformed address in MAIL FROM or RCPT TO, or a bad HELO name.
- Class
- 5xx · Permanent failure
- Retried?
- No, the message bounces
- Defined in
- RFC 5321
What to do
- Check the sender and recipient addresses for typos, spaces and missing angle brackets.
- Make the HELO/EHLO name a real host name of your server.
How it appears in a bounce
501 Syntax error in parameters
Paste the full bounce into the decoder — it finds every code and recipient and says what to check.