This has been happening to me for quite sometime, and is very unpredictable when it happens. When it happens, I click on X, then PM shows the blank mail with partially downloaded icon so I click on it to force download.
Interestingly, in my case, it has been 100% Spam mails that hangs, but the source code of the mail itself doesn't show any suspicious code nor attachment. -- - Hiro [PROTECTED] <[PROTECTED]> <[PROTECTED]>