550 5.1.10: Recipient not found (Microsoft 365)
At a glance
- Type: permanent failure (class 5, RFC 3463 §2). The sending server does not retry; sending the same message again fails the same way until the cause is fixed.
- Basic code 550: “Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)” (RFC 5321 §4.2.3).
- Enhanced code 5.1.10: “Recipient address has null MX” in the IANA registry (RFC 7505), in the group addressing status (RFC 3463 §3.2).
- Who uses it: Microsoft (Exchange Online) uses it for a recipient that is not found; the IANA registry name is the null MX meaning of RFC 7505.
If the bounce came from Microsoft 365, it means the recipient address was not found: no mailbox, group or contact in that organization has it. The same digits have a different official meaning in the standards, so the text after the code tells you which one you have.
Two meanings for one code
Microsoft's use. Exchange Online returns 550 5.1.10 when its SMTP address lookup finds no recipient with that address (Exchange Online NDR reference). A typical line reads 550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup. In practice it plays the same role as 550 5.1.1 does at other providers.
The registered meaning. In the IANA enhanced status code registry, X.1.10 is "Recipient address has null MX", added by RFC 7505 §6. A null MX is a DNS record a domain publishes to say it accepts no mail at all. RFC 7505 §4.1 says a server rejecting a recipient for that reason should use the basic code 556, not 550. That case has its own page: 556 5.1.10.
So: 550 with RecipientNotFound is a missing mailbox at a Microsoft-hosted domain. 556 with a mention of null MX is a domain that takes no mail.
Common causes
- A typo in the part before the @.
- The person left and their mailbox was removed.
- An alias or secondary address was taken off the mailbox it used to point to.
- The address never existed, for example a guessed
firstname.lastname@pattern.
How to fix it
If you are the sender:
- Check the spelling of the address in the bounce.
- Confirm the current address with the person through another channel.
- In Outlook, remove the address from autocomplete so it is not reused by mistake.
If you run the receiving Microsoft 365 organization:
- Search for the address among your mailboxes, groups, contacts and their alias addresses.
- If it should exist, add it to the right recipient and ask the sender to try again.
- If the address was retired on purpose, no action is needed; the bounce is working as intended.
What to do with the address
Treat a Microsoft 5.1.10 as a hard bounce: suppress the address and do not retry it. If several contacts at the same company bounce this way at once, the company may have moved its mail or changed its address format, so ask one of them for the new pattern before you remove them all.
Related codes
- 550 5.1.1: The recipient mailbox does not exist
- 556 5.1.10: The recipient domain publishes a null MX: it takes no mail
- 550 5.5.0: Mailbox unavailable at Outlook.com or Hotmail
- 5.4.1: Recipient address rejected: access denied (Microsoft 365)
See every bounce code in the dictionary.
Many bounces from addresses that do not exist can be avoided: check an address with our free email checker, or clean a whole list before you send.
How we wrote this: drafted by the email-validation.org team with the help of AI writing tools, from our product documentation and the sources linked in the text.