I've still got the "-1 unread" messages error. Interestingly it now turns my "In Tray" icon bold when stating there is a minus message unread. When I do have real unread messages the "In Tray" remains plain.
The error seems to have returned after a message was forwarded and deleted by an auto rule. I know how to fix the error, but I'll have to do so every time a message is auto deleted... -- david.gordon

