550 5.1.10: recipient not found (Microsoft 365)
Microsoft 365 couldn't find the recipient's address. It's usually a typo or an old address, but it can also be backscatter from mail you never sent.
What 550 5.1.10 means
Microsoft's NDR reference describes 5.1.10 as "Recipient not found": SMTP address lookup didn't find the recipient's address. It also lists 550 5.5.0 "Requested action not taken: mailbox unavailable" for hotmail.com and outlook.com addresses that aren't found, and calls it similar to 550 5.1.10.
Note that the standard meaning of X.1.10 is different. In the IANA registry, X.1.10 means "Recipient address has null MX", used with reply code 556. Microsoft uses 5.1.10 for a recipient that doesn't exist. Read the text to know which you have.
Microsoft's article gives two reasons. Either the recipient's address doesn't exist or couldn't be found, or you're seeing backscatter: a spammer sent mail to a non-existent address with your address forged as the sender, and the bounce came to you. Microsoft says backscatter is harmless and can be deleted, but if you get a lot, run an anti-malware scan and ask your admin to set up SPF. For real messages, check the address, clear the Auto-Complete entry, and ask the recipient to check forwarding rules.
Likely causes
- A typo or outdated recipient address.
- A bad entry in your Outlook Auto-Complete list.
- The recipient's forwarding rule sends to an address that doesn't exist.
- The recipient's mailbox doesn't exist or has no license.
- Backscatter: your address was forged on spam sent to invalid recipients.
How to fix it
If you sent the message
- Check the address and resend it typed fresh, after deleting the Auto-Complete entry.
- Compare the address in the NDR with the one you sent to. If they differ, the recipient has a forwarding rule to fix.
- If you never sent the original, treat it as backscatter.
If you're the recipient or their admin
- Confirm the recipient exists and has an active license.
- Review inbox rules and account forwarding that may send to a broken address.
Frequently asked questions
I got a 550 5.1.10 for an email I didn't send. Was I hacked?
Usually not. Microsoft explains this is often backscatter: a spammer forged your address on mail to a non-existent recipient. Check your Sent folder; if it's clean, delete the bounce. If you get many, scan for malware and make sure your domain has SPF.
Is Microsoft's 5.1.10 the same as the standard 5.1.10?
No. The IANA registry defines X.1.10 as a recipient domain with a null MX (with reply code 556). Microsoft uses 5.1.10 for a recipient address it can't find.
Sources
- Fix NDR error "550 5.1.10" in Exchange Online (Microsoft Learn)
- Email nondelivery reports (NDRs) and SMTP errors in Exchange Online (Microsoft Learn)
- IANA: SMTP Enhanced Status Codes registry
Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.
Related codes
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