Add timeout to java mail transport

                props.put("mail.smtp.connectiontimeout", "5000");
                props.put("mail.smtp.timeout", "5000");

            try
            {
                //Set the host smtp address
                Properties props = new Properties();

                if (Integer.parseInt(port) == 465) // SSL
                {
                   props.put("mail.smtp.auth", "true");
                   props.put("mail.smtp.socketFactory.port", port);
                   props.put("mail.smtp.socketFactory.class", "javax.net.ssl.SSLSocketFactory");
                   props.put("mail.smtp.socketFactory.fallback", "true");
                }
                else
                {
                   props.put("mail.smtp.starttls.enable", "true");
                }

                props.put("mail.smtp.connectiontimeout", "5000");
                props.put("mail.smtp.timeout", "5000");

                Session mailSession = Session.getInstance(props);
                mailSession.setDebug(false);
                Transport transport = mailSession.getTransport("smtp");
                MimeMessage message = new MimeMessage(mailSession);

                message.setSubject(subject);
                message.setContent(messageString, "text/plain");
                message.addRecipient(Message.RecipientType.TO,  new InternetAddress(recipients));
                message.setFrom(new InternetAddress(recipients));

                transport.connect(host, Integer.parseInt(port), username, password);
                transport.sendMessage(message, message.getRecipients(Message.RecipientType.TO));
                transport.close();
            } catch (Exception e)
            {
                return lastError;
            }

More info on javamail options
https://javamail.java.net/nonav/docs/api/com/sun/mail/smtp/package-summary.html