> I don't know of any technical reason to delay emails to minor domains. My domain has valid MX records, uses SPF, has valid DKIM TXT records, etc.

I still run into that with my business address; after much mucking around with client's MS admin who did various pieces of magic on their online/azure/microsoft email platform until we finally got it down to around 2m for the delay.

The upside appears to be that now all clients who are using microsoft appear to have only a 2m delay when sending email to my business domain.