550 5.1.1: user unknown (mailbox does not exist)
The receiving server says there is no mailbox with that name at the domain.
Permanent failure (hard bounce)
What 550 5.1.1 means
The domain exists and accepts mail, but the part before the @ is not a real mailbox. Servers phrase it as "User unknown", "Recipient address rejected" or "The email account that you tried to reach does not exist". It is a permanent failure: retrying will not help.
Common causes
- A typo in the address (jon@ instead of john@)
- The person left the company or closed the account
- The address was made up at signup
- An old list where mailboxes have since been deleted
How to fix it
- Check the address for typos and correct it
- Ask the recipient for a current address
- Remove the address from your list; do not retry
What to do with the address
Remove it. Repeated sends to non-existent mailboxes are the fastest way to damage your sender reputation. Verifying a list beforehand catches these addresses before you send.
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 550 5.1.1 mean?
- The receiving server says there is no mailbox with that name at the domain. The domain exists and accepts mail, but the part before the @ is not a real mailbox. Servers phrase it as "User unknown", "Recipient address rejected" or "The email account that you tried to reach does not exist". It is a permanent failure: retrying will not help.
- Is 550 5.1.1 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 550 5.1.1?
- Remove it. Repeated sends to non-existent mailboxes are the fastest way to damage your sender reputation. Verifying a list beforehand catches these addresses before you send.