SMTP codes
SMTP 500 Syntax error, command unrecognized
The server did not understand the command. Often a firewall or antivirus mangling the SMTP session.
- Class
- 5xx · Permanent failure
- Retried?
- No, the message bounces
- Defined in
- RFC 5321
What to do
- Check the client’s SMTP settings.
- Disable SMTP inspection on the firewall (Cisco “fixup smtp”, antivirus mail scanning).
How it appears in a bounce
500 Syntax error, command unrecognized
Paste the full bounce into the decoder — it finds every code and recipient and says what to check.