The default retry values for the spool are
1, 5, 5, 15, 30, 30, 30, 30, 60, 90, 120, 240, 480, 960, 1440, 2880
which means to have the message spooled for 4.45 days if there is a problem with delivery to the target smtp server
In this period, the sender thinks that he has sent the message and that it has been delivered
I personally have changed into
1, 5, 5, 15, 30, 30, 30, 30, 60, 90, 120, 120, 120, 120, 120, 180, 240, 360, 480, 600
so as not to exceed 2 days before sending the bounce message to the sender
But I believe that further implementation would also be needed.
Message Worning to sender after xx hour in spool
Like:
4 hours have passed and I am trying to deliver the xxxx message but I have not succeeded yet, I will keep trying.
And maybe repeat it every xx hours the worning, until the attempts are finished in which case the classic bounce is sent to the sender