Making M365 and Postmark share a domain (SPF, DKIM, Return-Path)
A domain can receive human email through Microsoft 365 and send transactional email through Postmark at the same time. The key is that only receiving needs MX.
Point MX at Microsoft so your shared mailbox gets human mail. Postmark only needs a DKIM record and a Return-Path CNAME (pm-bounces.yourdomain → pm.mtasv.net) — no MX — so the two coexist.
The one rule to respect: a domain may have only one SPF record. Merge both senders into it: v=spf1 include:spf.protection.outlook.com include:spf.mtasv.net -all.
inboxroot generates all of this for you per domain and checks it live, so you publish once and watch each record go green.