Alright, so I figured out the reason behind the e-mail problems. Basically, the /tmp directory on the linux box filled up. This was caused by one of my clients using the "Antivirus" checkbox. My /tmp directory was filled with "clamav" junk. Once I removed all files prefixed with "clamav", spamassassin started working properly again.
Is there a way I can prevent users from checking this box, or is there a method I can implement that will fix this from filling up my /tmp directory? Thanks.