5.1.0 bounce: other address status
5.1.0Other address statusPermanent: retrying will not help
550 5.1.0 <user@example.com>: Address rejected
What it means
X.1.0 is the RFC 3463 code "Other address status". The subject digit 1 says the trouble lies with an address, either the envelope sender (MAIL FROM) or a recipient (RCPT TO), and the detail digit 0 says the server did not pick a more precise reason such as 5.1.1 (no such mailbox) or 5.1.8 (bad sender domain).
Some Exchange and Microsoft 365 bounces use 5.1.0 for sender address problems, for instance a From or envelope address the server does not consider valid. Other systems use it as a generic "address not accepted" answer when they do not want to reveal whether the mailbox exists.
A 4.1.0 version exists for temporary address checks, usually when the receiving server could not finish a lookup (a directory query or a callout to your server) in time. That one is retried. A 5.1.0 is a final answer for this attempt.
Likely causes
- The envelope sender is empty, malformed, or uses a domain the receiving server will not accept.
- An Exchange connector or transport rule refuses the sender address, often after a mailbox rename or a domain removal.
- The recipient address belongs to a domain the server hosts but fails a validation step it chose not to describe.
- Sender address verification (a callout back to your MX) failed or returned an unexpected answer.
How to fix it
- Work out which address was refused. The bounce normally quotes it; compare it with the sender and recipient you meant to use.
- If it is your sender address, confirm the domain exists, has MX records, and that the address is spelled exactly as configured on your mail server or sending service.
- If you send through Exchange or Microsoft 365, check that the mailbox still owns that address and that the domain is still an accepted domain in the tenant.
- If it is the recipient address, verify it with the person directly rather than guessing variations, then resend once.
- If your server performs sender callouts and you receive these from your own users, test that the remote MX answers RCPT for the sender address.
Checks that apply
- MX Lookup Confirms the sender and recipient domains both publish working MX records, which many address checks require.
- Email Header Analyzer Reveals the actual envelope and header addresses used, which often differ from what the mail client showed.
- SMTP Diagnostics Tests whether the receiving server answers on port 25 and how it greets you before the address checks run.
FAQ
- Why did I get 5.1.0 instead of 5.1.1?
- The server chose the generic address code. Some systems avoid 5.1.1 on purpose so they do not confirm which mailboxes exist, and Exchange often uses 5.1.0 for sender problems.
- Can the recipient’s admin fix a 5.1.0 for me?
- Yes, if their server rejected a valid address by mistake. Give them the bounce time and message ID so they can find the rejection in their logs.