part1.setHeader("foo", "bar");
    public void testWriteTo() throws MessagingException, IOException {        MimeMultipart mp = new MimeMultipart();        MimeBodyPart part1 = new MimeBodyPart();        part1.setContent("Hello World", "text/plain");        mp.addBodyPart(part1);        MimeBodyPart part2 = new MimeBodyPart();        part2.setContent("Hello Again", "text/plain");        mp.addBodyPart(part2);        mp.writeTo(System.out);    }