Most of you may already know that Mail can display another window (the Activity Viewer) with per-task information about what Mail is doing while sending and receiving your mail. You can also end individual tasks (for example, disconnecting from an IMAP server that you know to be inaccessible). This window can be enabled via Command-0 or by picking Window -> Show Activity Viewer.
A third method of activitating the Viewer window is to option-click on the spinning sunflower progress indicator in the upper right corner of Mail.app's status bar.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020930055533736