I haven&#x27;t thought about what&#x27;s reasonable (an hour? a week?) but it still shouldn&#x27;t be dropped but bounced back to the sender with an undelivered message. At last that&#x27;s how email solved it some thirty years ago.<p>(But perhaps a new protocol could take the opportunity to improve things further and offer on-line delivery status notifications throughout the process.)<p>No matter what, messages should never be silently dropped.