sendsend.email
Start building

Free tools › SMTP error codes

5.1.1 Bad destination mailbox address

The part before the @ does not exist at that domain. This is the standard “user unknown” bounce.

5.1.1 Bad destination mailbox address

Permanent: retrying will not help

The part before the @ does not exist at that domain. This is the standard “user unknown” bounce.

Usual causes
  • A typo in the address, such as gmial.com or a missing letter.
  • The person left, and their mailbox was deleted.
  • An address that was never real.
What to do
  • Check the address for typos and correct it.
  • If it is right, remove it from your list and do not send to it again. Sending again to addresses that do not exist is one of the fastest ways to damage your sending reputation.
  • Check addresses when people sign up, to catch typos before the first email.

Usually the fault of: recipient.

Common questions.

What does 5.1.1 mean?

The part before the @ does not exist at that domain. This is the standard “user unknown” bounce.

Will retrying fix 5.1.1?

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 5.1.1?

Check the address for typos and correct it. If it is right, remove it from your list and do not send to it again. Sending again to addresses that do not exist is one of the fastest ways to damage your sending reputation. Check addresses when people sign up, to catch typos before the first email.

Other codes: 5.1.2 · 5.1.3 · 5.1.6 · 5.2.1 · 5.2.2 · 5.2.3 · 5.3.4 · 5.4.1 · 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