Microsoft 365 / Outlook
Microsoft 365 error 5.7.23 — SPF validation failed
550 5.7.23
Microsoft's own guidance for this code points specifically to an SPF configuration issue in the sending organization's Microsoft 365 setup — most often a missing or incorrect SPF include for Microsoft's own sending infrastructure, or a record that doesn't cover the actual sending path.
How to fix it
- 1
Verify your SPF record includes Microsoft's range
For Microsoft 365 mail flow, your SPF record needs include:spf.protection.outlook.com. Check with the SPF checker.
- 2
Check for a second SPF record
A domain migrated onto Microsoft 365 sometimes ends up with a leftover SPF record from a previous provider — a domain can only have one, and two makes SPF fail entirely.
- 3
Confirm the sending path matches
If mail is relayed through a third-party service before or after Microsoft 365, that service's IPs or include also need to be in the same SPF record.
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.