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.