a couple of days ago we stopped receiving mail and I checked the spam filter.  For the previous 45 minutes every log entry contained "No Data Received".  Also, I noticed that the number of incoming connections was stuck at 75 and never changed.  Simply restarting the service fixed everything.

we are running 2.6.3.476

any ideas?

By now you probably have restarted the service or program which is step one.  That message means that either you have a huge number of probes connecting to your server or the server is not processing messages quick enough and the sending servers are timing out the connections.

Next I'd watch to see what the avg processing time is for each message. SpamFilter is fast, but if too many checks or some lists are too large and your load is large, it could be that your server just can't handle the processing load.  You can try reducing the checks that you do, for example, disable bayesian filtering.  I'd also suggest a later build, at least 478 if not 481.