[Krei-se]

It's not reasonable. Mail not deliverable is not the same as house burned down, recipient moved unknown or sth, it simply means the letter was not received. Who and why messed up is unknown, thus NO mail server will mark you down after a single attempt.

Host your own!!

[toast0]

Reasonable and plausible are different things. I wouldn't be surprised if some outgoing servers just never get around to sending retries.