sendsend.email
Start building

Free tools › SMTP error codes

500 Syntax error, command unrecognized

The server did not understand a command it was sent. Mail software normally never triggers this.

500 Syntax error, command unrecognized

Permanent: retrying will not help

The server did not understand a command it was sent. Mail software normally never triggers this.

Usual causes
  • A badly written mail client or script talking SMTP by hand.
  • A line in the message was too long or contained characters the server does not accept.
What to do
  • Use a mail library or an email API instead of writing SMTP commands yourself.
  • Check the message for very long lines and unusual characters, and keep lines under 998 characters.

Usually the fault of: sender.

Common questions.

What does 500 mean?

The server did not understand a command it was sent. Mail software normally never triggers this.

Will retrying fix 500?

No. It is a permanent failure, so retrying the same message will not help, and repeated attempts can hurt your sending reputation. Fix the cause first.

How do I fix 500?

Use a mail library or an email API instead of writing SMTP commands yourself. Check the message for very long lines and unusual characters, and keep lines under 998 characters.

Other codes: 250 · 421 · 450 · 451 · 452 · 501 · 503 · 530 · Decode a whole bounce message

See why every email bounced.

SendSend shows each message’s result, and the reason the receiving server gave. Free for 3,000 emails a month.

Get started freeCheck your domain