Exchange 5.5 has been out of support and reached EOL years ago. I do not know of anything available today that still supports Exchange 5.5. The latest Exchange is 2010 and 2007 is also still available. I do not know if there is a direct upgrade pat from 5.5. Both those versions of Exchange require a 64-bit OS to run on. Both have built in spam filters which work fairly well, and you can also add your favorite RBL’s to assist in blocking spam.
From what you are saying, it sounds like the client machines are generating spam and passing it through Exchange. If this is the case, you need to thoroughly scan the client machines fior malware, and remove that which is found.
If the spam is incoming from the Internet and being distributed by the server, you may want to try SpamBayes, an open source product which supports Outlook back to the 2000 version. It is decent from loading, but it does take about two weeks to train it well, at which time your users are likely to see very little, if any, spam in their Inboxes.