Microsoft 365 5.1.10
Recipient not found
Microsoft 365 5.1.10 means: recipient not found. It is a permanent failure, so do not retry it unchanged. Treat it as a hard bounce: check the address for a typo, then remove it from the list.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
Recipient not found
Possible cause
SMTP address lookup didn't find the recipient's <SMTP Address>.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.1.10: Recipient address has null MX |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | Yes, verification catches this |
In practice
550 5.1.10 RESOLVER.ADR.RecipientNotFound is the Exchange Online reply for an address it cannot find by SMTP lookup. It is the Microsoft 365 equivalent of an unknown-user bounce, and the address should be treated as a hard bounce. Microsoft also sends it when a spammer forged your address as the sender and the bounce went back to you, so a sudden wave of 5.1.10 bounces for messages you never sent points to spoofing, not to your list. Check the address for typos first. Microsoft tenants do not accept near matches. If a whole batch of 5.1.10 bounces comes from one company domain, the company may have moved to a new domain or tenant, so check its website before deleting the contacts.
What the sender should do
Treat it as a hard bounce: check the address for a typo, then remove it from the list.
Would verifying the list have prevented Microsoft 365 5.1.10?
Yes, verification catches this
A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.
Common questions
- What does Microsoft 365 5.1.10 mean?
- Microsoft 365 5.1.10 means: recipient not found. It is a permanent failure, so do not retry it unchanged. Treat it as a hard bounce: check the address for a typo, then remove it from the list.
- Should I retry after Microsoft 365 5.1.10?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. Treat it as a hard bounce: check the address for a typo, then remove it from the list.
- Would verifying my list have prevented Microsoft 365 5.1.10?
- Yes, verification catches this. A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.
Sources
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05