Microsoft 365 / Outlook
Outlook.com error 550 5.7.515 — sender domain authentication failed
550 5.7.515
Outlook.com rejected the message because the domain in the visible From header didn't pass Microsoft's authentication checks — a hard rejection tied to Microsoft's equivalent of Gmail/Yahoo's sender requirements, which extended enforcement to high-volume senders sending to Outlook.com in 2025.
How to fix it
- 1
Verify SPF and DKIM both pass
Check with the SPF checker and DKIM checker — Microsoft's requirements, like Gmail's and Yahoo's, expect both authenticated for high-volume senders.
- 2
Confirm DMARC alignment
SPF or DKIM passing isn't enough on its own — the authenticated domain needs to align with the visible From header domain. See the alignment guide if SPF/DKIM pass but this still bounces.
- 3
Publish a DMARC record if you don't have one
A DMARC record, even at p=none, is part of the baseline these rules expect. Generate one with the DMARC generator.
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.