Email error 550 5.1.1: User unknown (mailbox does not exist)

What it means

The receiving server says the recipient mailbox does not exist. It is a permanent failure (a hard bounce), so retrying the same address will never work.

Common causes

• The address has a typo, for example a misspelled name or domain.
• The person left the company or closed the account.
• The mailbox never existed because the address was guessed or made up.
• The address came from an old list and has since been deleted.

How to fix it

1. Remove the address from your list; do not retry it.
2. Check the spelling and, if the person is known, ask for a new address.
3. Look at the full bounce message: it often names the exact address that failed.
4. Verify the rest of the list before the next send.

How to prevent it

Remove the address from your list and verify the rest before the next campaign.

Most bounces come from addresses that were never checked. Verify each address before you send: syntax, domain, mailbox and risk are checked without sending a message. Test a single address with the free email checker or upload a whole list for bulk verification.

Verify an email address free · Clean a whole list · MX lookup · SPF, DKIM and DMARC checker

Frequently asked questions

Is 550 5.1.1 a permanent error?

Yes. The 5 in the code means permanent. Providers expect senders to stop mailing the address.

Can verification prevent it?

Yes. A verifier asks the receiving server whether the mailbox exists and flags it before you send.

Other error codes

All email error codes