First of all, may you have a look at "First Impressions of Thunderbird 2.0" by the houstonmacbro. Recently I found a HTTP mail plug-in for Mail (the e-mail client come with Mac OS). After couple of hours playing with it, the Mail get wierd. It no longer able to download e-mail. So I've removed it.
One day, my boss asked me to help him on "a urgent matter" - his Mail is not working. He know there is some new messages from server but the Mail just stay in the "downloading" status. Unlike Outlook or Outlook Express, it does not have a pop-up window when downloading e-mails. There is nothing an end-user would able to do when it is not working. Since all his business e-mails are there, he is very frustrated. Since I've read something about the Thunderbird 2.0, I recommended him NOT to upgrade to Thunderbird. After couple of tries, quits and restarts, we believe the problem was the plugin and so we disabled it. Da-da, it's working again!
So, seems that plug-ins for Mail does not work at all. That's it.