Common causes
- The mailbox does not exist and the server is set to refuse unknown recipients.
- The address is valid but only accepts mail from internal or approved senders.
- A directory lookup on the receiving side blocked the sender.
- The domain recently moved mail systems and the old mailboxes were not carried over.
How to fix it
- Treat the address as unreachable unless you know it should work.
- If the person is a known contact, ask for another way to reach them.
- Check whether the domain changed its mail provider.
- Do not retry in a loop; one retry later is enough.
How to prevent it Take the address off your list and verify the rest before the next send.
Frequently asked questions
Is it the same as 550 5.1.1?
Close, but not identical. 5.1.1 says plainly that the mailbox is unknown, while 5.4.1 says access was denied, which can also be a policy.
What does a verifier do with it?
It reads the full reply. If the wording clearly says the recipient does not exist, the address is Invalid; if it only denies access, the result is not guessed.