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Which senders are covered?
The policy applies to Microsoft’s consumer email service, Outlook.com, including messages sent to outlook.com, hotmail.com, and live.com addresses. Microsoft defines a high-volume sender as a domain sending more than 5,000 emails per day. The threshold is based on the sending domain, not an individual recipient’s mailbox.
Microsoft published the requirements on April 2, 2025, updated the announcement on April 30, and confirmed May 5, 2025, as the enforcement start date in its Outlook.com Postmaster announcement. This is a consumer Outlook.com policy; it does not automatically establish the same rule for every Microsoft 365 or Exchange Online tenant.
What authentication must the domain have?
Microsoft requires SPF and DKIM to pass, and DMARC to be configured with alignment to either SPF or DKIM. The mechanisms work together; they are not alternatives from which a sender can choose just one.
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- SPF: Check that the domain’s SPF record authorizes the servers or services that send its mail.
- DKIM: Ensure outbound messages are signed and that the signatures validate.
- DMARC: Publish a DMARC record and verify that the authenticated SPF or DKIM identity aligns with the message’s visible From domain.
Microsoft’s requirements announcement describes the authentication and alignment rules. A DNS record’s presence alone is not enough: the checks must pass on mail actually sent.
What happens when a sender does not comply?
Microsoft says non-compliant messages are initially sent to Junk. It may later reject them until the sender corrects the relevant DNS records. Enforcement began May 5, 2025, so a sender encountering this policy should treat authentication failures as an operational issue rather than assume the message will simply arrive in the inbox.
Why can correctly authenticated mail still go to Junk?
Passing SPF, DKIM, and DMARC addresses authentication requirements, but does not guarantee inbox placement. Microsoft says its filtering also considers IP and domain reputation, list accuracy, complaints, and message content. Its sender guidance identifies junk complaint rate as a principal factor in reputation and deliverability.
Microsoft Support explicitly cautions that submitting information through its delivery-support process does not guarantee that messages will be delivered. The Outlook.com sender support guidance recommends identifying the sender clearly, moderating sending volume, and avoiding mail to people who never read or reply. Senders should also monitor complaint rates, keep lists accurate, and assess their IP and domain reputation and content.
How to troubleshoot a delivery problem
- Verify authentication in real outbound mail. Confirm that SPF authorizes the actual sending infrastructure, DKIM signatures validate, and DMARC aligns with SPF or DKIM.
- Check sending practices. Review list accuracy, remove unengaged or invalid recipients, and avoid abrupt or excessive volume changes.
- Review reputation signals. Monitor complaints and investigate issues involving the sending IP, domain, or message content.
- Use Microsoft’s sender support process when appropriate. Provide clear sender identification and relevant delivery details, while recognizing that a support submission is not a delivery guarantee.
Microsoft’s support page also identifies optional third-party tools for spam database checks, Sender Score, and Return Path Certification. These are not Microsoft requirements or endorsements, and Microsoft says it is not responsible for third-party site content.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Recipient-side safe senders are not a sender fix
An Outlook.com recipient can add a trusted sender or domain through Microsoft’s Safe Senders instructions, which can reduce the chance of that sender’s mail landing in Junk for that recipient. This personal setting does not replace sender authentication compliance or assure delivery to all recipients.
For Microsoft 365 administrators, visible From and SMTP MAIL FROM addresses can differ in bulk mail, and safe sender lists inspect the visible From address. Microsoft Learn warns against broad allowlisting that bypasses spam filtering because it can enable spoofing or impersonation. Those tenant-side controls are distinct from Outlook.com’s high-volume sender authentication policy: Microsoft Learn: Create allowlists.
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