I create a message:msg = new MimeMessage(session);MimeBodyPart mbp1 = new MimeBodyPart();mbp1.attachFile(f1);MimeBodyPart mbp2 = new MimeBodyPart();mbp2.attachFile(f2);MimeBodyPart mbp3 = new MimeBodyPart();mbp3.attachFile(f3);Multipart mp = new MimeMultipart();mp.addBodyPart(mbp1);mp.addBodyPart(mbp2);mp.addBodyPart(mbp3);msg.setContent(mp);...then send it:SMTPTransport transport = (SMTPTransport)session.getTransport();transport.connect();transport.sendMessage(msg, new Address[] {
   new InternetAddress(...)});and get MessagingException: IOException while sending messageUnsupportedDataTypeException: no object DCH for MIME type multipart/mixed: There is something wrong with MailCap, javamail can not find a handler for such multipart/mixed part.  I solved this issue by adding this (before mail creation and delivery)             MailcapCommandMap mc = (MailcapCommandMap) CommandMap.getDefaultCommandMap();             mc.addMailcap("text/html;; x-java-content-handler=com.sun.mail.handlers.text_html");             mc.addMailcap("text/xml;; x-java-content-handler=com.sun.mail.handlers.text_xml");             mc.addMailcap("text/plain;; x-java-content-handler=com.sun.mail.handlers.text_plain");             mc.addMailcap("multipart/*;; x-java-content-handler=com.sun.mail.handlers.multipart_mixed");             mc.addMailcap("message/rfc822;; x-java-content-handler=com.sun.mail.handlers.message_rfc822");             CommandMap.setDefaultCommandMap(mc); Note that JAF comes bundled in Java >= 6