The first time the MAIL$MESSAGE_GET routine is called, the
    message information is returned for the first requested message,
    and the status returned is MAIL$_MSGINFO. Subsequent calls to
    MAIL$MESSAGE_GET with the MAIL$_MESSAGE_CONTINUE item code return
    the message text records with the status MAIL$_MSGTEXT, until no
    more records are left, when MAIL$_NOMOREREC is returned.