550 5.1.10: Microsoft 365 recipient not found
550 5.1.10Recipient not found by SMTP address lookupPermanent: retrying will not helpFrom Microsoft 365 and Outlook.com
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup
What it means
Exchange Online’s resolver looked the recipient address up in the tenant directory and found nothing: no mailbox, mail user, contact, group, or proxy address carries it. The tenant accepts mail for the domain, so the domain part is fine; only the local part is unknown.
Microsoft uses 5.1.10 where many other servers would say 5.1.1. The practical difference is that 5.1.10 comes from a directory lookup after the message reached Microsoft, while a 5.4.1 means directory based edge blocking refused the address during the SMTP conversation before any mailbox resolution.
For external senders this almost always means a typo or a departed employee. Inside a tenant it often points to a cached Outlook autocomplete entry that still holds an old LegacyExchangeDN or an address that was removed.
Likely causes
- The address was mistyped, for example a missing letter or the wrong domain suffix.
- The recipient left the organisation and the mailbox, or the alias you used, was deleted.
- Outlook autocomplete offered a stale entry that points to an object which no longer exists.
- A hybrid or migrated mailbox lost a proxy address during migration, so the old address no longer resolves.
How to fix it
- Sender: check the spelling of the address character by character, then try a known working address at the same company.
- Sender using Outlook: delete the suggestion from the autocomplete list and type the address again in full.
- Recipient admin: search the Microsoft 365 admin center for the address, including aliases, shared mailboxes, and groups.
- Recipient admin: if the person left, add the old address as an alias on a shared mailbox or the manager’s mailbox so mail keeps arriving.
- After a migration, compare proxyAddresses on the cloud object with the on-premises object and add any missing ones.
Checks that apply
- MX Lookup Confirms the recipient domain points at Microsoft 365, so you know the 5.1.10 came from the right tenant and not an old mail host.
- Typosquat Scan Spots look-alike or misspelled domains that resolve to a different tenant than the one you meant.
- MailFlow Monitor Sends a test message to the corrected address and confirms it now lands.
FAQ
- Is 5.1.10 the same as 5.1.1?
- Both mean the mailbox does not exist. 5.1.10 is the code Exchange Online uses after a directory lookup, so treat it the same way: fix the address, do not resend.
- Will the sender retry?
- No. It is a 5xx permanent failure, so the message is returned at once.
- The address works for some senders but not others. Why?
- Internal senders often hit a cached autocomplete entry. Clearing that entry usually fixes it while external senders were never affected.