Mail Server Compromised?

[ASAI]

Greetings,

In the logs I have been seeing many attempts made to send messages to 
gmail which seem like there's spam being sent from my server.  In the 
logs I see this:

Dec 24 00:05:11 triata amavis[29729]: (29729-06) Passed CLEAN, 
<apa...@triata.globalchangemultimedia.net> -> <ickovjulee...@gmail.com>, 
Message-ID: 
<20101224070510.bf7acfd8...@triata.globalchangemultimedia.net>, mail_id: 
s69xqJA1Kuer, Hits: -2.6, size: 669, queued_as: 9F457FD80A9, 898 ms
Dec 24 00:05:11 triata postfix/smtp[1065]: BF7ACFD8063: 
to=<ickovjulee...@gmail.com>, relay=127.0.0.1[127.0.0.1]:10024, delay=1, 
delays=0.09/0.01/0/0.9, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as 
9F457FD80A9)

What is a problem is that there is no user named apa...@triata... and 
this user is sending hundreds of emails out to Gmail.  So it looks like 
there's been a compromise.  My question is, how do I begin to plug this 
hole?