Permanent failure — do not retry
SMTP 550 5.1.10
recipient not found (Microsoft 365)
SMTP 550 5.1.10 is Exchange Online's "recipient not found" error: Microsoft looked up the recipient's address and found no mailbox for it. It is a hard bounce, like 5.1.1. Microsoft notes it can also be backscatter, when a spammer forged your address as the sender.
How it appears
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup
Remote Server returned '550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient user@example.com not found by SMTP address lookup'
What causes it
- The address has a typo or the mailbox was deleted.
- The user exists but has no licence or mailbox in the Microsoft 365 tenant.
- A forwarding rule on the recipient's side points at an address that no longer exists.
- Your address was spoofed, and you are seeing bounces for mail you never sent.
How to fix it
In order — try the first one first.
- Check the address and resend. In Outlook, remove the stale auto-complete entry first.
- If the bounce shows a different address than the one you sent to, tell the recipient their forwarding is broken.
- If you never sent the original message, check SPF, DKIM and DMARC on your domain to make spoofing harder.
- Otherwise remove the address. Retrying will not help.
Common questions
- What does SMTP 550 5.1.10 mean?
- SMTP 550 5.1.10 is Exchange Online's "recipient not found" error: Microsoft looked up the recipient's address and found no mailbox for it. It is a hard bounce, like 5.1.1. Microsoft notes it can also be backscatter, when a spammer forged your address as the sender.
- Should I retry a 550 5.1.10 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 5.1.10?
- Check the address and resend. In Outlook, remove the stale auto-complete entry first.
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 5.1.0 — address rejected, other address status
- 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