[logifail]

> I take that to mean that if your server isn't availble to receive the mail at the time it is first offered, it won't be retried later.

Umm, RFC 5321, which describes queuing and retry? SMTP is designed to be very forgiving of transient network issues.

> That wasn't the case (for most mail) when I gave up on self hosting 10 years ago, but it's plausible

Plausible? To those of us who run our own mailservers, the OP's statement is an extraordinary claim.