Microsoft Exchange Server 03″ Spam

I work for a moderately sized public library in the Systems Department. For a while now, our Microsoft Exchange Server 2003 has been sending gratuitous amounts of spam.
I’m sure yesterday, my supervisor and I made trips to the closet housing our firewall to restart it atleast 8 times due to being overloaded.
Ridiculous!

Our traffic will remain moderately bogged, while the load is usually almost maxed out. But depending on how many pc’s are actively online and using the Internet, it easily shoots the load up to maxing out.

Any ideas for methods to get rid of it or control the amount of blocked spam more effectively?