Most outbound setups run into an SPF problem nobody talks about:
The 10 DNS lookup limit. Every new tool adds includes, and if you go over 10, SPF fails even if everything looks fine. That can break DMARC alignment and now Gmail or Microsoft may just reject the message. Lesson: check your lookup count every time you add a sender.
Source: r/u/Kamm-E · by /u/Kamm-E