550 5.1.10 Microsoft bounce: recipient not found
What 550 5.1.10 means
Exchange Online's SMTP address lookup did not find the recipient's address, so the message cannot be delivered.
Message text
Recipient not found
Common causes of 550 5.1.10
- Address does not exist in the recipient organization
- Recipient domain is hosted in Exchange Online but the address is not a valid recipient
How to fix 550 5.1.10
- Verify the address exists and re-type it.
- Run MX lookup on the recipient domain to confirm which mail system receives its mail.
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How to read 550 5.1.10
The first part, 550, is the SMTP reply code. Requested action not taken, mailbox unavailable: the mailbox does not exist, is not allowed, or the message was refused for policy reasons. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
The second part, 5.1.10, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 5 is a Permanent failure (5xx): the command was rejected and the sender should not repeat it unchanged (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 1 is addressing: a problem with the recipient or sender address. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Do not keep resending the same message: a permanent failure repeats until the cause is fixed. Correct the cause, then send a new message.
Official source: learn.microsoft.com. Last verified 2026-10-20.
Related guides
- Null MX (RFC 7505): publish it for domains with no mail
- How to set up MX records (priorities, null MX, migration)
- MX host does not resolve: causes and how to fix it
Related codes
- 5.1.10 SMTP: recipient has null MX
- 5.1.1 SMTP: bad destination mailbox
- 5.1.2 SMTP: bad destination system
- 5.1.3 SMTP: bad address syntax
- 550 5.1.1 Microsoft: bad mailbox address
- 5.4.1 Microsoft: relay access denied
FAQ
What does 550 5.1.10 mean?
Exchange Online's SMTP address lookup did not find the recipient's address, so the message cannot be delivered. This is a permanent failure.
How do I fix 550 5.1.10?
Verify the address exists and re-type it. Run MX lookup on the recipient domain to confirm which mail system receives its mail.
Is 550 5.1.10 permanent, and should I retry?
550 5.1.10 is a permanent failure, so retrying the same message will fail again. Fix the cause first, then send a new message.