By default, a message that is not deliverable will stay in the Postfix queue for 5 days with the sender not being notfied at all during this time.
Such a setting may have been useful in the days the internet wasn’t as reliable as it is today, but hardly makes sense anymore.
To change this, edit
and add or alter the following parameters:
maximal_queue_lifetime = 2d
delay_warning_time = 6h
This will reduce the maximum time a message stays in the queue to 2 days, while at the same time, notifying the sender of the delay after 6 hours.
When done issue:
sudo postfix reload