queue notices

[Jon Rust]

I've seen a few mail systems notify users that mail hasn't been
delivered when it's been queued for X number of days, but hasn't yet
expired.  Say your queuelifetime is set to 1 week. After a message
hasn't delivered for 1 day, let the sender know that it hasn't and also
that you (the mail server) will keep trying for another 6 days.

Has anyone seen a patch like this? Any thoughts on implementing the idea?

Thanks,
jon