Need help securing mail server: already got hacked.

[John Foster <jfoster@augustmail.com>]

I have exim4 using the heavy daemon set up and it runs fine. I am setting it 
up to serve mail for 5 sites (all my own) that I have on a single box using 
Apache2 and as virtual sites. All is working fine . I was advised that I 
should set up a pop3 server in order to allow access from a remote system to 
these mail sites. Again (I used popa3d)  everything worked fine. The problem 
is, I got hacked almost immediately and my mail system was used to send a 
load of spam all over the place. The result is that I have had to shut down 
the servers until I get my system hardened enough to do the job & quit 
getting hijacked by spammers. I have been reading the docs but time is not my 
friend here I need to get this done ASAP so I can get these servers turned 
back on. My ISP will shut me down if I leave the system up in its current 
state. All tips greatly appreciated.
Thanks!
-- 
John Foster