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Run email for many domains on one Microsoft 365 license
Microsoft bills per license, not per domain. Here’s how free shared mailboxes let you give every domain its own inbox in Outlook — and where Postmark fits for transactional mail.
Making M365 and Postmark share a domain (SPF, DKIM, Return-Path)
You can receive human mail through Microsoft and send transactional mail through Postmark on the same domain. The trick is one merged SPF record and non-conflicting DKIM.