Retrieves the message text of messages specified by the MessageSet property.
def fetch_message_text() -> None: ...
This method retrieves the RFC822-encoded text of the message specified by message_set. If the local_file property contains a file name, the text is stored in local_file, and the on_transfer events denote the progress. If the local_file property contains an empty string, the text is stored in the message_text property, as well as provided through the on_transfer event.