One other option is that you have used up your capacity on your e-mail server. I had this problem once and had no indication other than the exclamation point. It turned out that there was a preferences that was set to maintain the message on the server even after it was downloaded to the inbox. So, even though the inbox was cleaned out, all of the old e-mails were still on the server.
Check how your account is set up accordng to instructions here:
http://ca.huji.ac.il/services/intern...xpresspc.shtml
Who is the e-mail account with?