Microsoft 365 / Outlook
Microsoft 365 error 550 5.7.1 — message blocked by a security policy
550 5.7.1
This NDR indicates a security setting — in your own Exchange organization or the recipient's — is preventing delivery. It's a broad code that covers several underlying causes: a mail flow (transport) rule, a spam-confidence-level threshold, or the recipient's anti-spam policy rejecting the message outright.
How to fix it
- 1
Check your own transport rules
In the Exchange admin center, review mail flow rules for anything that could be blocking outbound mail matching this pattern.
- 2
Check authentication
Unauthenticated or misaligned mail is treated with more suspicion by anti-spam policies on both sides. Verify SPF, DKIM, and DMARC are all correctly published and aligned.
- 3
Ask the recipient to check their side
If you control the sending domain and authentication is clean, the block may be on the recipient's tenant — a transport rule or blocked-sender entry on their end that only they can see or remove.
- 4
Check reputation
Run the blacklist checker on your sending IP — this NDR can also surface when a recipient's spam filter treats the sender as high-risk.
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.