It started after my 10.6.3 update.
I'm basically looking for a "Clear cache" button in Mail. It won't fetch new Gmail (big problem), and there are six copies of the same message in my Inbox that won't delete (spits out the following error).
I want to know if (a) anyone else has noticed this issue or (b) found a solution for a similar problem.
It's worth it to note that I was getting the dreaded Finder -10810 Won't Start error, along with the progressive lose of control. This prompted two (2) hard resets of the machine and then I immediately went and installed this update. Has not happened since the update but now it's the mail issue.