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So I recognize that I've been horribly remiss in not upgrading to a newer version, but, since I was at 5.0.16, I just disabled ClamAV, using the instructions in the sticky thread, and mail seemed to be working ok.
I plan to update this Saturday night, when traffic is light.
In the mean time, no email with attachments are making it through, and even uploading an attachment causes the Web UI to freeze up.
Bugzilla
Wiki
Source
ClamAV diabled, now no attachements get through 
Linear Mode
