A generic address error. Depending on the wording it refers to the recipient (address not accepted) or to your sender address.
Hard bounce · permanent
Common causes
- Recipient address not accepted by the server
- Your From / envelope sender address is not valid or not allowed
- Sending from a domain that has no MX or A record
How to fix it
- Read the text after the code — it usually says whether the sender or the recipient is the problem
- Make sure your sending domain has valid MX records and can receive replies
- Verify the recipient address
Bounce ExplainerRun the right check for your domain or paste the full bounce message for a precise explanation.
Open Bounce Explainer →Frequently asked questions
Is 5.1.0 the same as 5.1.1?
5.1.1 specifically means the mailbox does not exist. 5.1.0 is a broader "other address status" code.