Permanent failure — do not retry
SMTP 500
syntax error, command unrecognised
SMTP 500 means the receiving server did not recognise the command it was sent. RFC 5321 defines it as "syntax error, command unrecognized", and it also covers command lines that are too long. It is a fault in the client software or something in the path, not a problem with the recipient address.
How it appears
500 Syntax error, command unrecognized
500 5.5.2 Error: bad syntax
500 5.5.1 Command unrecognized
What causes it
- A misbehaving or custom SMTP client sent a malformed or unsupported command.
- A command line was longer than the server accepts.
- A firewall or antivirus product rewrote the SMTP session in the middle.
- Plain text was sent over a port that expects TLS from the first byte, or the other way round.
How to fix it
In order — try the first one first.
- Check the SMTP client or library configuration, especially the port and TLS mode (implicit TLS on 465, STARTTLS on 587).
- Disable any SMTP inspection on firewalls or antivirus software between you and the server and test again.
- Capture the SMTP session log to see which command was rejected.
- Do not suppress the recipient. Nothing about the address is implied by a 500.
Common questions
- What does SMTP 500 mean?
- SMTP 500 means the receiving server did not recognise the command it was sent. RFC 5321 defines it as "syntax error, command unrecognized", and it also covers command lines that are too long. It is a fault in the client software or something in the path, not a problem with the recipient address.
- Should I retry a 500 error?
- No. 500 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 500?
- Check the SMTP client or library configuration, especially the port and TLS mode (implicit TLS on 465, STARTTLS on 587).
Last reviewed 2026-09-11