5.1.0: address problem, unspecified
Something about an address in the message, the sender's or a recipient's, made the server refuse it, without saying exactly what was wrong.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.1.0
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
5.1.0 Other address status
What it means
5.1.0 belongs to the addressing group (X.1.x), which covers the sender and recipient addresses given in the SMTP envelope. The detail 0 means the server did not narrow the problem down: the address could be unknown, malformed, or refused by a local rule. As a 5.y.z code it is permanent and the message is returned; a 4.1.0 would ask the sender to try again later and is rarely seen. RFC 3463 notes that addressing errors can generally be corrected by the sender, so the first suspect is the address itself, then the receiving server's rules for addresses.
Common causes
- A recipient address that does not exist or is misspelled, reported without the more precise 5.1.1.
- A sender address the receiving server will not accept, for example at a domain that cannot receive replies.
- An address refused by a local rule the server does not describe.
How to fix it
- 01Check both the envelope sender and the recipient address in the bounce for typos.
- 02Look up the recipient domain's MX records to confirm it receives mail.
- 03Make sure your sender domain exists and has working MX records, so replies and bounces can reach it.
- 04If both addresses look right, ask the recipient's administrator which address was refused and why.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.1.1Mailbox does not exist.The recipient's domain accepts mail, but the mailbox named before the @ does not exist there, so the message was refused and will not be delivered.
- 5.1.8Sender domain cannot receive mail.The domain in the sender address does not exist or cannot accept mail, so the receiving server refuses a message whose bounces could never be returned.
- 550Mailbox unavailable.The receiving server permanently refused the message for this recipient: the mailbox does not exist, cannot be reached, or the server rejected the message by policy.
5.1.0 at other providers
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce