I did it, here's how I did it in case anyone else needs it:
Edit /etc/mail/virtusertable:
xxx@mydomain.com xxx
aa@mydomain.com aa
@mydomain.com gossamermail

(let's say we have users xxx,a and we want the gossamermail account to catch the rest)

#cd /etc/mail
#make all

Edit /etc/sendmail.cw, add the line:
mydomain.com

Restart sendmail, and it should all be working.
(Note: This is for RH 6.2, with sendmail installed from the RH cd)