The DeleteMessage method marks the specified message for
deletion. If the optional message number is not specified, then the
current message is deleted. Once a message has been marked as
deleted, any attempt to access it will result in an error.
The message will not actually be removed from the server until
the Disconnect method is called or the InternetMail object
is unloaded. To prevent messages which have been marked for
deletion from actually being removed from the mailbox, call the
Reset method.
This method will return value of zero if the action was
successful. Otherwise, a non-zero error code is returned which
indicates the cause of the failure.