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.
Should you remove the address?
Remove the address from your list
Yes. 5.1.10 means the tenant has no such recipient. Unless you can find a typo, the address is dead and should come off the list.
Hard bounce. Do not retry. It will fail every time.
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 [email protected] 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.
Who sends it
Microsoft 365 and Exchange Online tenants, which covers a large share of business mail. You will often see it with the text RESOLVER.ADR.RecipientNotFound.
Microsoft 365
Provider's description SMTP address lookup didn't find the recipient's <SMTP Address>.
Microsoft names this NDR "Recipient not found".
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.
- Is 550 5.1.10 a hard bounce or a soft bounce?
- 550 5.1.10 is a hard bounce. The receiving side has said the address can't receive mail, so a retry won't deliver it. Do not retry. It will fail every time.
- Should I remove the address after a 550 5.1.10 bounce?
- Yes. 5.1.10 means the tenant has no such recipient. Unless you can find a typo, the address is dead and should come off the list.
- Will verifying my list prevent 550 5.1.10?
- Yes. A mailbox check asks the recipient's server whether the address exists without sending anything, so addresses that would return this code are flagged before the send instead of bouncing against your domain.
Clean the list before the next send.
A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.
- A real SMTP mailbox check on every row, before anything is sent
- Dead and mistyped addresses split out, so they never reach your sending tool
- Catch-all, disposable and role addresses flagged alongside
Plans from $9 a month, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.
More Microsoft replies: every Outlook and Microsoft 365 SMTP error, and the 550 5.7.515 bulk sender rules.
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
- 550 5.5.0 — Mailbox unavailable (Outlook.com, Hotmail)
- 550 5.7.12 — Sender not authenticated by organization (Microsoft 365)
- 550 5.7.513 — Blocked by the recipient's own block list (Microsoft 365)
Sources
- Source: Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online
- Source: RFC 3463: Enhanced Mail System Status Codes
Provider wording is quoted only where the provider publishes it.
Last reviewed 2026-10-22