I haven't thought about what's reasonable (an hour? a week?) but it still shouldn't be dropped but bounced back to the sender with an undelivered message. At la

I haven't thought about what's reasonable (an hour? a week?) but it still shouldn't be dropped but bounced back to the sender with an undelivered message. At last that's how email solved it some thirty years ago. (But perhaps a new protocol could take the opportunity to improve things further and offer on-line delivery status notifications throughout the process.) No matter what, messages should never be silently dropped.