mAIb Deliverability Repair

Outlook.com sender requirements

Microsoft enforces authentication for high-volume senders to Outlook.com, Hotmail and Live.com by rejecting at SMTP rather than filtering — which means the mail bounces rather than landing anywhere a recipient could find it.

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What Microsoft requires

For senders above roughly 5,000 messages a day to Microsoft's consumer mailboxes:

  • SPF must pass.
  • DKIM must pass.
  • A DMARC record with at least p=none.
  • DMARC alignment with either SPF or DKIM.

550 5.7.515

If you are seeing bounces reading 550 5.7.515 Access denied, sending domain [domain] does not meet the required authentication level, that is this enforcement. Microsoft's approach differs from filtering in an important way: non-compliant mail is rejected at the SMTP conversation, so it bounces back to you rather than going quietly to a junk folder. That is inconvenient, but it is far more visible than silent filtering, and the bounce names the problem.

Working through it

Check all four in order. SPF and DKIM both have to pass — not one or the other. Then confirm a DMARC record exists; p=none is sufficient for the requirement. Then confirm alignment, which means the domain a recipient sees in From: matches the domain that authenticated. Alignment is the one that catches people, because a platform sending on your behalf often authenticates as its own domain rather than yours.

Alignment cannot be checked from DNS. Send one message to a mailbox you can read and check its Authentication-Results header, or paste the headers here.

Source: Microsoft's published high-volume sender requirements, retrieved 2026-09-14.

Common questions

Why is my mail bouncing from Outlook.com but delivering to Gmail?

Microsoft rejects non-compliant mail at SMTP where other providers filter it. The same authentication gap that puts a message in a Gmail spam folder produces a hard bounce at Outlook.com.

Does this apply to Microsoft 365 business mailboxes?

The published high-volume requirements are about Microsoft’s consumer services — Outlook.com, Hotmail.com and Live.com. Authentication matters for business mailboxes too, but this specific enforcement targets the consumer side.

Find, explain, repair, verify

The scan is free and complete. If it finds something wrong, the £19 Repair Pack gives you the exact records to publish for your domain and your providers, the order to publish them in, and the previous values so you can undo anything.

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Ruleset 2026.09.14, sources verified 2026-09-14. Methodology.