Retrieves the message text of messages specified by the MessageSet property.
The 'callback' parameter specifies a function which will be called when the operation completes (or an error is encountered). If the 'callback' parameter is not specified, then the method will block and will not return until the operation completes (or an error is encountered).
The callback for this method is defined as:
'err' is the error that occurred. If there was no error, then 'err' is 'null'.
'err' has 2 properties which hold detailed information:
This method retrieves the RFC822-encoded text of the message specified by MessageSet. If the LocalFile property contains a file name, the text is stored in LocalFile, and the Transfer events denote the progress. If the LocalFile property contains an empty string, the text is stored in the MessageText property, as well as provided through the Transfer event.