Me too!
I redirect email that goes to the addresses we have for support, accounting and sales back through a gmail account then forward the gmail account to a second email I created for those accounts. This allows me to filter for spam as well as filter out the daemons and other bounce backs we receive in the gmail account. This works very well and catches about 99% of it.
I've been doing the same thing for the last 5 years. It's been an excellent way to filter SPAM.
Recently switching to osTicket has caused our reponses to emails to come from the second email address that Gmail forwards back to, which is not what we wanted.
So I created a mod to allow a different Reply From address (our original email address) to be sent from. Check it out: http://osticket.com/forums/showthread.php?t=7756(http://osticket.com/forums/showthread.php?t=7756)