Transport.send(msg) 不起作用

我正在尝试使用 javamail 将电子邮件从一个帐户发送到另一个帐户。但代码无法执行 Transport.send(msg);线。可能的原因是什么？ 下面是下面的jsp代码。

<%

        String host = "localhost";
        String to = request.getParameter("to");
        String from = request.getParameter("from");
        String subject = request.getParameter("subject");
        String messageText = request.getParameter("body");
        boolean sessionDebug = false;

        Properties props = System.getProperties();
        props.put("mail.host", host);
        props.put("mail.transport.protocol", "smtp");

        Session mailSession = Session.getDefaultInstance(props, null);

        mailSession.setDebug(sessionDebug);

        try { 
        Message msg = new MimeMessage(mailSession);

        msg.setFrom(new InternetAddress(from));
        InternetAddress[] address = {new InternetAddress(to)};
        msg.setRecipients(Message.RecipientType.TO, address);
        msg.setSubject(subject);
        msg.setSentDate(new Date());
        msg.setText(messageText);

        Transport.send(msg);

        out.println("Mail was sent to " + to);
        out.println(" from " + from);
        out.println(" using host " + host + ".");
        } catch (MessagingException mex) {mex.printStackTrace();}
    %>

这些是我经常使用的属性

properties.put("mail.transport.protocol", "smtp");
properties.put("mail.smtp.host", host);
properties.put("mail.user", from);
properties.put("mail.smtp.port", smtpPort);
properties.put("mail.smtp.localhost", "myHost");

Session session = Session.getInstance(properties, null);

你会注意到我设置了mail.smtp.localhost如果机器主机名未正确设置，则需要设置该值，例如对于虚拟机等