The Mail pane shows messages currently in the mail queue waiting to be sent.
Summarised in the title bar at the top of the window is the total number of messages currently in the queue as well as the current status of the Mailer.
The Mailer can be stopped and started using the buttons at the bottom of the window or through the pop-up menu which is shown when the pane is right clicked. You would stop the mailer if you need to change an email message for some reason before it is sent, or if you want to perhaps delete a message from the queue.
Entries in the list can have their To: From: and Subject: fields edited in-line should the need be necessary. The retry state may also be reset on failed messages by selecting this option from the right click popup menu.
The same right click pop-up menu also contains options for customising the display allowing the user to turn on and off various columns.
Entries may be deleted by selecting them and then choosing the Delete option from the right click pop up menu.
If the State of a message cannot be sent due to some kind of error, you can reset the state by selecting the message and choosing the Reset State function. This makes the message available to be sent by the mailer again.