email virus problems

I am a full time volunteer with a non profit organization in Australia. I do a good portion of the IT here (with another volunteer), however we have had little training for it. Much of what we know is from doing it. Our network runs Terminal Services on Windows Server 2003, we use Microsoft Antispyware on all machines, as well as Norman Anti-Virus installed on the network.

We bought a Barracuda email spam filter about 1.5 years ago and it has been going well for us for some time. Recently, however, with the release of the sober virus, we have been having some problems. Our organizations office is mixed with both desktops and personal laptops being used for work purposes. So once we realized that we had sober, we did virus checks on all work machines, personal machines, and all servers, cleaning all of them. This seemed to fix it, but after returning from a recent trip I have found that our spam count has gone through the roof, while a lot of spam is getting through somehow.

During the sober virus problem time, our mail server was blacklisted causing more problems. So we requested that be re-evaluated after we clear the virus. Now, I have found that the Barracuda Spam Filter caught 74,000 spam emails coming from the IP of our own Linux mail/SME server. The emails were all blocked, but I am also finding that some have been going through on previous days, saying they are bounce emails being sent by our email server that are being ''bounced'' (possibly because of the blacklisting of our server?).

What are my possibilites here? Is there an virus on our linux box? Could someone have re-infected there laptop and it got back into our system? We have had no red flags come up from our virus systems at all. From what I know of how sober works, it just finds the closest email server and sends spam out through there. I just don't have enough information and understanding to know what the exact problem is and how to fix it.