What it looks like in your logs
550 5.1.0 <user@example.com>: Address rejectedWhat does 5.1.0 mean?
RFC 3463 reserves X.1.0 for address problems that do not fit a more specific code. A 5.1.0 tells you that the receiving server found something permanently wrong with an address in the transaction, but it has not committed to saying whether the mailbox is unknown, the domain is bad or the syntax is off. It is the address-class equivalent of a shrug.
Well-behaved servers prefer the precise codes: 5.1.1 for an unknown user, 5.1.2 for a bad destination domain, 5.1.3 for bad syntax. When a server falls back to 5.1.0, the human-readable text is where the detail lives. Some servers also attach 5.1.0 to a rejected sender address rather than a recipient, so check which address the text names before you suppress anything.
Exchange and some hosted filters use 5.1.0 in address-rejected and sender-denied messages, and some appliances fall back to it when a directory lookup fails for reasons they do not report. Gmail and Yahoo rarely use it and prefer 5.1.1 for unknown recipients. If the same text repeats across many recipients at one domain, the problem is more likely routing on their side than bad addresses on yours.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 5.1.0 | ✓ this code |
Common causes
- The recipient address does not exist, and the server reports it generically.
- The recipient domain resolves but its mail routing is broken.
- The sender address was rejected by a filter or policy that uses 5.1.0.
- The address has a formatting problem the server did not classify further.
How to fix 5.1.0
- Read the text after the code to see whether the sender or recipient address was rejected.
- If it is the recipient, suppress it and check for obvious typos in the domain.
- Check the recipient domain has working MX records with the MX lookup.
- If it is your sender address, confirm the From and envelope addresses are valid, published domains that can receive replies.
- Validate addresses at signup to keep generic address failures out of the list; our list hygiene guide explains how.
5.1.0 questions
What does bounce code 5.1.0 mean?
It means the receiving server permanently rejected an address without giving a specific reason code. It sits in the address class, so the problem is with an address rather than your content or reputation. Read the bounce text for details.
How do I fix a 5.1.0 "Address rejected" bounce?
Work out which address was rejected. If it is the recipient, remove it from your list and check the domain with the MX lookup. If it is your own sender address, make sure it uses a real domain with valid DNS and that your server is allowed to send as it.
Is 5.1.0 the same as 5.1.1?
They are close but not identical. 5.1.1 specifically means the mailbox does not exist. 5.1.0 is the catch-all for address problems, which often includes unknown users but can also cover routing or sender issues. Treat both as hard bounces. Neither should be retried.