The OnProgress event is generated when a message is being
retrieved or sent. This event can be used to update the user
interface, such as displaying a progress bar during the
transaction. To cancel the current operation, the application can
call the Cancel method from within this event.
The MessageSize argument is a long integer which
specifies the size of the message in bytes that is currently being
sent or received.
The MessageCopied argument is a long integer which
specifies the number of bytes that have been sent or received for
the current message.
The Percent argument is an integer which specifies the
completion percentage between a value of 0 and 100.