使用JavaMail发送电子邮件时发生MailConnectException

Question

我正在尝试按照本教程发送带有JavaMail的电子邮件： http://www.tutorialspoint.com/java/java_sending_email.htm

这是我的代码：

    public void sendAccountActivationEmail()
    {
        String to = "myemail@gmail.com";
        String from = "myemail@gmail.com";
        String host = "localhost";

        Properties properties = System.getProperties();
        properties.setProperty("mail.smtp.host", host);
        Session session = Session.getDefaultInstance(properties);

        try{
            MimeMessage message = new MimeMessage(session);
            message.setFrom(new InternetAddress(from));
            message.addRecipient(Message.RecipientType.TO, new InternetAddress(to));
            message.setSubject("Account activation");
            message.setContent("<h1>This is actual message</h1>", "text/html" );
            Transport.send(message);
            System.out.println("Sent message successfully....");
        }catch (MessagingException mex) {
            mex.printStackTrace();
        }
    }

然而，当运行此方法时，我得到以下异常：

com.sun.mail.util.MailConnectException: Couldn't connect to host, port: localhost, 25; timeout -1;
  nested exception is:
    java.net.ConnectException: Connection refused: connect
    at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:2100)
    at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:699)
    at javax.mail.Service.connect(Service.java:366)
    at javax.mail.Service.connect(Service.java:246)
    at javax.mail.Service.connect(Service.java:195)
    at javax.mail.Transport.send0(Transport.java:254)
    at javax.mail.Transport.send(Transport.java:124)
    at com.myproject.managed.UserRegistrationController.sendAccountActivationEmail(UserRegistrationController.java:161)
    ...

Caused by: java.net.ConnectException: Connection refused: connect
    at java.net.TwoStacksPlainSocketImpl.socketConnect(Native Method)
    at java.net.AbstractPlainSocketImpl.doConnect(Unknown Source)
    at java.net.AbstractPlainSocketImpl.connectToAddress(Unknown Source)
    at java.net.AbstractPlainSocketImpl.connect(Unknown Source)
    at java.net.PlainSocketImpl.connect(Unknown Source)
    at java.net.SocksSocketImpl.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at com.sun.mail.util.SocketFetcher.createSocket(SocketFetcher.java:331)
    at com.sun.mail.util.SocketFetcher.getSocket(SocketFetcher.java:238)
    at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:2066)
    ... 58 more

我该怎么办？