Microsoft 365 / Outlook
Microsoft 365 error 550 5.1.10 — recipient not found (often backscatter)
550 5.1.10
The recipient address wasn't found in the destination organization. If you received this bounce for a message you don't recall sending, Microsoft's own guidance identifies a specific pattern: a spammer forged your address as the From/Return-Path on spam sent to a non-existent recipient, and the resulting bounce landed on you — a pattern called backscatter. That's safe to ignore or delete.
How to fix it
- 1
If you sent the message: check the address
Confirm the recipient's email address doesn't have a typo and that the mailbox still exists.
- 2
If you didn't send it: it's likely backscatter
This is a symptom that your domain is being spoofed, not a delivery problem you caused. Check your DMARC policy — a stronger policy (quarantine or reject) makes it harder for spoofed mail using your domain to be accepted anywhere in the first place, which reduces backscatter over time.
- 3
Verify your own DMARC enforcement
Run the DMARC checker. If your policy is still p=none, this is a concrete reason to move toward enforcement.
Meaning current as of 2026-07-14, cross-checked against the provider's own documentation. Providers do occasionally redefine codes — if this doesn't match what you're seeing, the source link is the authoritative reference.