550 5.1.10 error (Exchange Online): Recipient not found
RESOLVER.ADR.RecipientNotFound - Microsoft 365 couldn't find a mailbox, group or contact for this address.
How to fix 550 5.1.10
- Check the spelling and the domain, or remove the address.
- If it should exist, the recipient's admin has to create or restore it.
Why this happens
The receiving server looked for the mailbox and couldn't find one it will accept. Typos, people who left a company, and lists that were scraped or are years old are the usual sources.
How to keep it from coming back
Verify addresses before you send and remove hard bounces immediately. Keep your bounce rate under about 2%; a list that produces many of these damages the reputation of the domain you send from.
Should I retry a 550 5.1.10 error?
No. This is a permanent failure for that address, so retrying only adds another bounce to your count. Remove the address from your list, or correct it if it's a typo.
Got the full bounce message?
The text after the code names the exact reason, and the same code can mean different things at different receivers. Paste the whole message into the decoder - it runs in your browser and nothing is sent.
Open the bounce decoderRelated error codes
Seeing this again and again? That's usually a reputation or authentication problem we can trace and fix at the source.
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