The receiving server says the recipient address does not exist. This is a permanent failure (hard bounce); retrying will not help.
Hard bounce · permanent
Common causes
- A typo in the address (e.g. gmial.com, missing letter)
- The person left the company and the mailbox was deleted
- The address never existed — fake sign-ups or old purchased lists
- The domain moved to a new mail provider and the user was not created there
How to fix it
- Check the address for typos and confirm it with the recipient another way
- Remove the address from your list — repeated sends to unknown users damage your sender reputation
- Verify lists before sending so invalid addresses never reach a campaign
- If it is your own domain, make sure the mailbox exists at your current mail provider
Inbox CheckerRun the right check for your domain or paste the full bounce message for a precise explanation.
Open Inbox Checker →Frequently asked questions
Should I retry a 550 5.1.1 bounce?
No. It is permanent. Remove the address; mailbox providers watch how many unknown users you hit.
Can a 5.1.1 be wrong?
Rarely — some servers use it for disabled or blocked accounts. If an important contact bounces, ask them for an alternative address.