550 5.1.10 RESOLVER.ADR.RecipientNotFound: Microsoft 365's 'Not Found' Bounce and What to Do With It
Treat 550 5.1.10 as a hard bounce: the address doesn't exist in that Microsoft 365 organization, so suppress it. The same goes for 550 5.5.0 "mailbox unavailable" from outlook.com and hotmail.com. Microsoft's own page for 5.1.10 is mostly written for admins fixing their own tenant; for someone sending to a list, there are only two cases where the address might still be good.
Sources, read 2 November 2026: Microsoft Learn's Fix NDR error 550 5.1.10 in Exchange Online (updated August 2026) and Email nondelivery reports and SMTP errors in Exchange Online.
Microsoft's "not found" codes side by side
| Code | Text | Microsoft's meaning |
|---|---|---|
| 550 5.1.10 | RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup | The recipient's address doesn't exist or couldn't be found |
| 550 5.5.0 | Requested action not taken: mailbox unavailable | outlook.com/hotmail.com address not found by SMTP address lookup; similar to 5.1.10 |
| 550 5.1.1 | Bad destination mailbox address | Mistyped address, address doesn't exist, or a moved mailbox |
| 550 5.4.1 | Recipient address rejected: Access denied | Address doesn't exist; rejected at the edge by Directory-Based Edge Blocking |
The 5.4.1 version gets its own post: 550 5.4.1 access denied. Code pages: 550 5.1.10 and 550 5.5.0.
Why B2B lists produce so many
People leave companies and their mailboxes are removed, but their addresses stay in lead databases and CRMs. When a Microsoft 365 tenant can't find the address, it says 5.1.10. A handful per send is normal job-change decay; a lot from one campaign means the list is old or was built by guessing address formats.
The two exceptions
- The bounce names a different address. Compare the recipient in the bounce with the one in your sent message. If they differ, the person probably has an inbox rule, account forwarding or an admin mail-flow rule sending your mail to a dead address. Microsoft says the recipient (or their admin) has to fix or remove the rule. The original address may be fine; contact the person another way if it matters.
- You never sent the message. If the bounce is for mail you didn't send, it's backscatter from a spammer forging your From address. Microsoft says it's safe to ignore and doesn't mean your account was compromised. Publishing SPF and a DMARC policy makes your domain harder to forge.
Step by step for a list sender
- Collect the 5.1.10 and 5.5.0 bounces from the send; paste them into the bounce message analyzer to separate them from temporary failures.
- Check for the two exceptions above; keep those addresses aside.
- Suppress the rest in the sending tool and remove them from the CRM or source list.
- Verify the remaining Microsoft-hosted contacts before the next send. The Microsoft 365 email checker is built for company Outlook addresses, or upload a whole list on the verify page.
Honest limits
Microsoft is one of the harder providers to check ahead of a send. Many Microsoft 365 tenants accept the recipient during a check and only reject later, or answer the same way for every address, so a share of results comes back risky or unknown rather than invalid. That's covered in why Outlook leads come back risky. Verification still removes a lot of dead addresses; it can't promise zero 5.1.10s.
Where SimpleVerifier fits
Verify a B2B list on the verify page before it goes out. The first 100 checks are free with a Google sign-in. Pro is $39 a month with the first 100,000 checks a month at full speed, then slower, never cut off. See pricing.
Common questions
What does 550 5.1.10 RESOLVER.ADR.RecipientNotFound mean?
Microsoft's Exchange Online documentation says the recipient's email address doesn't exist or couldn't be found: SMTP address lookup didn't find it in the receiving Microsoft 365 organization. For a sender, it's a permanent failure for that address.
Should I remove an address that bounced with 550 5.1.10?
Yes, in almost every case. Suppress it in your sending tool and in the source list. The only time to keep it is when the bounce names a different address than the one you sent to, which points to a broken forwarding rule on the recipient's side.
Is 550 5.5.0 mailbox unavailable the same as 5.1.10?
Microsoft's NDR table lists 550 5.5.0 'Requested action not taken: mailbox unavailable' for hotmail.com and outlook.com recipients not found by SMTP address lookup, and describes it as similar to 550 5.1.10.
Why did I get 5.1.10 bounces for emails I didn't send?
That's backscatter: someone sent spam to non-existent recipients with your address forged in the From field, and the bounces came to you. Microsoft says it's safe to ignore and doesn't mean your account is compromised; publishing SPF and DMARC makes spoofing your domain harder.
Check your list before you send
Real mailbox checks over SMTP, and nothing is sent to the address. Sign in with Google and your first 100 email checks are free, no card. Plans for bigger lists are on the pricing page.