When an e-mail user on my domain uses Squirrel (or NeoMail in the past) and mistypes the "To:" address in a message she is sending, the error message from <Mailer-Daemon@scorpio.lunarpages.com> saying:
SMTP error from remote mailer after RCPT TO:<email@example.com>:
host mxb.earthlink.net [126.96.36.199]: 550 firstname.lastname@example.org...User unknown
is NOT addressed to the e-mail user who sent the message, but rather it is sent to the domain main account - i.e. it is sent to me. This is a big problem because:
 I an getting tired of forwarding these messages to the original senders, and
 when I'm backpacking in the wilderness the person sending the e-mail won't know she mistyped the address of the person she was planning to sell a bridge to next day, until I'm back at my computer two weeks later.
Note that Horde does NOT make this mistake but my users seem to prefer the Squirrel.
I've asked this before - 66 people read the question but no-one responded. I think this is a big problem and I'm surprised it hasn't been resolved, or even discussed here. Maybe it has and I just haven't noticed.
P.S. the whole miss-addressed message gets returned to me which is kind of an invasion of the sender's privacy - I'll leave the details to your imagination