username+mailbox(_at_)domain - how to set up?

I want to use multiple mailboxes in my return address (as I see some people
on this list doing).  Can someone point me to the relevant docs on how to
set this up?  We are using sendmail 8.x with mail as the local delivery
method (I can't talk our sysadmin into using procmail as the local deliverer,
I have a feeling it would be much nicer).


TIA for any help,

Jason Spears