Nowadays, everyone think about e-mail as something must get delivered in a few minutes.
I know is not this way it was thinked about, but then was 20 years ago, when dial up rules!
In a business environment, do not have an e-mail delivered after one day is too much.
IMHO iIf there are any problem, it's better to know it was no delivered then wait 1 week to return.
So just after to find out about this timing, I change several of my SME install to 1 day timeout. One client even asked for a 4 hours timeout!
I (and my clients) know if there are ANY problem with our server, their servers or even on web, the e-mail will return sooner than it could... but everyone I talk appears to prefer this way.
I'll put those templating commands in wiki if nobody has nothing against it!
because I think it's a REALLY NICE FEATURE to be able to control in how much time your messages return if not delivered.