Permanent failure — do not retry
SMTP 550 5.1.0
address rejected, other address status
SMTP 550 5.1.0 means the receiving server rejected the address without giving a more specific reason. RFC 3463 defines X.1.0 as "other address status". Microsoft's Exchange Online reports it as "sender denied" when a recipient cannot be resolved. Treat it as a permanent address problem and check the recipient.
How it appears
550 5.1.0 Address rejected.
550 5.1.0 Sender denied
What causes it
- The recipient address does not resolve on the receiving server and the server did not say why.
- In Exchange Online, a reply was sent from an old or offline copy of a message whose recipient no longer resolves.
- The sender address itself is being refused by the receiving server.
How to fix it
In order — try the first one first.
- Check the recipient address for typos and resend to the correct address.
- In Outlook, type the address fresh rather than picking an old entry from the auto-complete list.
- If the address is right and it keeps failing, treat it as dead and remove it from the list.
Common questions
- What does SMTP 550 5.1.0 mean?
- SMTP 550 5.1.0 means the receiving server rejected the address without giving a more specific reason. RFC 3463 defines X.1.0 as "other address status". Microsoft's Exchange Online reports it as "sender denied" when a recipient cannot be resolved. Treat it as a permanent address problem and check the recipient.
- Should I retry a 550 5.1.0 error?
- No. 550 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 550 5.1.0?
- Check the recipient address for typos and resend to the correct address.
Other 550 replies
- 550 5.1.1 — mailbox does not exist
- 550 5.7.1 — message refused by policy
- 550 5.4.1 — recipient address rejected, access denied
- 550 5.2.1 — mailbox disabled, not accepting messages
- 550 — mailbox unavailable
- 550 5.1.10 — recipient not found (Microsoft 365)
- 550 5.1.8 — bad outbound sender (Microsoft 365)
- 550 5.4.5 — daily sending limit exceeded (Gmail)
- 550 4.4.7 — message expired in the queue
- 550 5.7.25 — no PTR record for sending IP (Gmail)
- 550 5.7.26 — blocked, sender unauthenticated (Gmail)
- 550 5.7.27 — SPF authentication failed (Gmail)
- 550 5.7.28 — unusual rate of unsolicited mail (Gmail)
- 550 5.7.23 — SPF violation (Microsoft 365)
- 550 5.7.124 — sender not in allowed senders list (Microsoft 365)
- 550 5.7.509 — DMARC failed, policy reject (Microsoft 365)
- 550 5.7.606-649 — banned sending IP (Microsoft)
- 550 5.7.705 — tenant has exceeded threshold (Microsoft 365)
- 550 5.7.708 — traffic not accepted from this IP (Microsoft 365)
- 550 5.7.750 — blocked from sending from unregistered domains (Microsoft 365)
- 550 5.7.515 — authentication required for high-volume senders (Outlook.com)
- 550 5.7.1 S3140 — IP range on Outlook.com block list
Last reviewed 2026-09-11