You will probably need to invest in some better technology or maybe even outsource this activity. My organization uses and is a huge fan of Postini (now owned by Google). False positives are very few and all of the filtering takes place “in the cloud” so the bad traffic never hits your network’s edge. This will allow you to use the network connection for other business requirements rather than having to contend with bad messages sharing the pipe with business related messages.
You don’t mention the specific solution you are using so it is difficult to say maybe it is an old version or what products might be better. Take a look at the SC Magazine IT Security Buyers Guide website for solutions that might meet your organization’s requirements.
In the IT trenches? So am I – read my IT-Trenches blog.