I've had several times where mail from custom domains associated with a business were delivered hours late by Outlook. Time sensitive emails. A guy coming to my office for a meeting called me and said he was downstairs and having an issue with security. Went down and swiped him in.

Four hours later, I got the same message by email. I thought he was back - nope. An email sent at 10am that was delivered at 4pm showed it was from 2pm until I looked at the header and saw sent at 10am and just... chilling out for 4 hours. DMARC was set up for them just fine.

I've had the same issue with my own custom domain, with DMARC set up. A year later, it finally got bumped up to some sort of "trusted" domain for that one place and my emails get delivered the same day now.