550 5.1.10 error: recipient not found
Microsoft's 550 5.1.10 means it couldn't find the recipient's address. Usually it doesn't exist; sometimes a broken forwarding rule on their side.
What 550 5.1.10 means
550 5.1.10 Recipient not found by SMTP address lookupMicrosoft looked the address up and found nothing. If the address in the bounce isn't the one you wrote to, the recipient forwards to a bad address.
List problem or sender problem?
List problem. Normally the address doesn't exist. Occasionally the recipient has a forwarding rule pointing at a bad address, and the bounce comes from that. It's a permanent failure: the server won't accept this message if you send it again unchanged.
How to fix 550 5.1.10
- Check the spelling, and resend once only if you corrected it.
- If the address in the bounce differs from the one you sent to, the recipient has a broken forwarding rule. Let them know another way.
- Remove addresses that keep returning 5.1.10.
- Bounces for mail you never sent are backscatter from a forged From address. Microsoft says you can ignore those.
In SendingOS, a 550 5.1.10 bounce shows its reason and code on the conversation, and the thread is labelled Bounced, so you can filter the inbox to bounces and pull these addresses before they add up.
Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.
Related codes
Sources: Microsoft: fix 550 5.1.10 · Microsoft: NDRs in Exchange Online
Questions
Is 550 5.1.10 the same as 550 5.1.1?
Close. Both mean the recipient wasn't found. 5.1.10 is Microsoft 365's version from its address lookup, and it can also come from a recipient's broken forwarding rule.