RFCs dictate that a mail server retry sending when the MX host is unavailable. If the sending mail server doesn't retry then it is the sender's fault if a messa

RFCs dictate that a mail server retry sending when the MX host is unavailable. If the sending mail server doesn't retry then it is the sender's fault if a message is returned undeliverable without retrying at least a few times. The "de facto standard" for retrying was (is?) five days, because that's what sendmail defaulted to for years. Nowadays, many senders don't retry that long (my mail systems are set for 48 hours) simply so that a bounce/undeliverable mail is returned to the sender much sooner. IIRC, there was a certain version of Exchange (I forget which now) that apparently didn't retry and would immediately bounce a message if it was undeliverable on the first attempt, but that was quickly fixed.