重用SMTP连接以通过java

Question

我正在使用Java Mail（1.4.7）发送电子邮件。我计划创建一个SMTP连接并重新使用它（通过保持活动状态）。我正在使用的代码如下所述。当我按预期运行程序时，我在日志中看到“已发送电子邮件”，但邮件未送达。此外，日志没有错误。如果我杀了程序（在eclipse上循环运行），邮件就会被传递。

或者，如果我在每个发送请求中关闭连接（通过调用smtpTransport.close（）），邮件将被传递（不会终止程序）。但这与为每个邮件发送请求创建新的SMTP连接一样好。

只是想知道，如果这可能是SMTP服务器问题？

private Session smtpSession; 
private Transport smtpTransport;
public synchronized void send(String toAddress, String ccAddress, String bccAddress, String emailSubject,
        String emailContent) throws MessagingException {
    // check if we need to create a new connection
    establishSmtpConnection();

    Message message = new MimeMessage(smtpSession);

    // set From: header field of the header.
    message.setFrom(new InternetAddress(smtpFromAddress));

    // set to recepients
    message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(toAddress, true));

    // set cc recepients
    if (ccAddress != null && !ccAddress.trim().equals("")) {
        message.setRecipients(Message.RecipientType.CC, InternetAddress.parse(ccAddress, true));
    }

    // set bcc recepients
    if (bccAddress != null && !bccAddress.trim().equals("")) {
        message.setRecipients(Message.RecipientType.BCC, InternetAddress.parse(bccAddress, true));
    }

    // set Subject: header field
    message.setSubject(emailSubject);

    // set the actual message
    Multipart multiPart = new MimeMultipart("alternative");

    // create the HTML message
    MimeBodyPart htmlPart = new MimeBodyPart();
    htmlPart.setContent(emailContent, "text/html; charset=utf-8");

    multiPart.addBodyPart(htmlPart);
    message.setContent(multiPart);

    Address[] allRecipients = message.getAllRecipients();

    LOGGER.info("Going to send email. message:{}, allRecipients:{}", message, allRecipients);

    message.saveChanges();

    // Send message
    smtpTransport.sendMessage(message, allRecipients);

    // ****** MAILS ARE DELIVERED INSTANTLY IF THE FOLLOWING IS UNCOMMENTED ************
    // smtpTransport.close();

    LOGGER.info("Email has been sent");
}

    /**
     * Establishes SMTP connection if not connected already.
     * @throws MessagingException
     */
    private synchronized void establishSmtpConnection() throws MessagingException {
        if (smtpTransport != null && smtpTransport.isConnected()) {
            LOGGER.debug("Will re-use exisitng SMTP connection..");
            return;
        } else {
            synchronized (MailUtil.class) { // double null check
                if (smtpTransport == null || !smtpTransport.isConnected()) {
                    // establish SMTP connection
                    LOGGER.info(
                            "Going to create a new SMTP connection. smtpHost:" + smtpHost + ", smtpPort:" + smtpPort);

                    Properties mailProps = new Properties();
                    mailProps.put("mail.smtp.host", smtpHost);
                    mailProps.put("mail.smtp.port", smtpPort);

                    smtpSession= Session.getInstance(mailProps, null);
                    smtpSession.setDebug(debug);

                    smtpTransport = smtpSession.getTransport("smtp");

                    LOGGER.info("Connecting to SMTP server...");

                    smtpTransport.connect();

                    LOGGER.info("Successfully connected to SMTP server...");
                }
            }
        }
    }