The AppendMessage method appends the specified string to
the end of the body of text in the current message part. Each line
of text contained in the string should be terminated with a
carriage-return/linefeed (CRLF) pair, which is recognized as the
end-of-line.
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.