500 5.5.1: syntax error, command unrecognized
The server did not understand a command your mail software sent.
Permanent failure (hard bounce)
What 500 5.5.1 means
This is a protocol-level fault, not a problem with the address. The sending software sent something the server did not recognise, such as a bad command or an unsupported extension.
Common causes
- A misconfigured mail client or script
- A proxy or security gateway rewriting the SMTP conversation
- Using the wrong port or encryption mode
How to fix it
- Check the port and encryption settings (587 with STARTTLS, or 465 with TLS)
- Update the mail software
- Test with a different client to isolate the fault
What to do with the address
Unrelated to the recipient address.
Check an address now with the free email checker, or look at a domain's mail setup with the email domain checker, MX lookup and SPF checker.
Related error codes
Frequently asked questions
- What does 500 5.5.1 mean?
- The server did not understand a command your mail software sent. This is a protocol-level fault, not a problem with the address. The sending software sent something the server did not recognise, such as a bad command or an unsupported extension.
- Is 500 5.5.1 a hard bounce or a soft bounce?
- Permanent failure (hard bounce). The address will keep failing, so remove it.
- What should I do with the address after 500 5.5.1?
- Unrelated to the recipient address.