spam city...

Hello All,

Lately my mail queue has been getting a much higher number of queued  
messages, mainly from spam. Apparently someone is sending spam and  
using my server as a return address. This has in turn stopped some of  
our legitimate mail from going through and some showing a reason as  
temporarily greylisted. I am running sendmail 8.12.9/ qpopper 4.0.5  
on a Mac G4, OS 10.2.8. If someone can please point me in the right  
direction I would appreciate it.

Jim