They said...
>> Email is supposed to be resilient to down time (retries, trying each MX record, etc.) but I found that large mail providers tend to just bounce and walk away.
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. That wasn't the case (for most mail) when I gave up on self hosting 10 years ago, but it's plausible.