JavaMail：无法连接到SMTP服务器

Question

The following code causes an error. Please help me understand what's wrong.

import javax.mail.*;
import javax.mail.internet.*;
import java.util.*;
public class SendMail
{
  public static void main(String [] args)throws MessagingException
  {
    SendMail sm=new SendMail();
     sm.postMail(new String[]{"abc@yahoo.co.in"},"hi","hello","xyz@gmail.com");
   }

public void postMail( String recipients[ ], String subject, String message , String from) throws MessagingException
{
    boolean debug = false;

     //Set the host smtp address
     Properties props = new Properties();
     props.put("mail.smtp.host", "webmail.emailmyname.com");

    // create some properties and get the default Session
    Session session = Session.getDefaultInstance(props, null);
    session.setDebug(debug);

    // create a message
    Message msg = new MimeMessage(session);

    // set the from and to address
    InternetAddress addressFrom = new InternetAddress(from);
    msg.setFrom(addressFrom);

    InternetAddress[] addressTo = new InternetAddress[recipients.length]; 
    for (int i = 0; i < recipients.length; i++)
    {
        addressTo[i] = new InternetAddress(recipients[i]);
    }
    msg.setRecipients(Message.RecipientType.TO, addressTo);


    // Optional : You can also set your custom headers in the Email if you Want
    msg.addHeader("MyHeaderName", "myHeaderValue");

    // Setting the Subject and Content Type
    msg.setSubject(subject);
    msg.setContent(message, "text/plain");
    Transport.send(msg);
}
}

Exception:
<pre>
com.sun.mail.smtp.SMTPSendFailedException: 450 smtpout04.dca.untd.com Authentication required

    at com.sun.mail.smtp.SMTPTransport.issueSendCommand(SMTPTransport.java:1829)
    at com.sun.mail.smtp.SMTPTransport.mailFrom(SMTPTransport.java:1368)
    at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:886)
    at javax.mail.Transport.send0(Transport.java:191)
    at javax.mail.Transport.send(Transport.java:120)
    at SendMail.postMail(SendMail.java:52)
    at SendMail.main(SendMail.java:10)

Answer 1

The "Authentication required" in the exception message suggests that the target SMTP server requires you to log in (Perhaps via TLS or SSL). This wasn't common on SMTP servers until a few years ago (it's an anti-spam measure) so it's easy to overlook.

To authenticate with JavaMail :

To use SMTP authentication you'll need to set the mail.smtp.auth property (see below) or provide the SMTP Transport with a username and password when connecting to the SMTP server. You can do this using one of the following approaches:

• Provide an Authenticator object when creating your mail Session and provide the username and password information during the Authenticator callback.

  Note that the mail.smtp.user property can be set to provide a default username for the callback, but the password will still need to be supplied explicitly.

  This approach allows you to use the static Transport send method to send messages.

• Call the Transport connect method explicitly with username and password arguments.

This approach requires you to explicitly manage a Transport object and use the Transport sendMessage method to send the message. The transport.java demo program demonstrates how to manage a Transport object. The following is roughly equivalent to the static Transport send method, but supplies the needed username and password:

Transport tr = session.getTransport("smtp");

tr.connect(smtphost, username, password);

msg.saveChanges(); // don't forget this

tr.sendMessage(msg, msg.getAllRecipients());

tr.close();

Answer 2

Note that many, many ISPs block access to external port 25 on servers outside of their network. Instead, the ISP forces you to use their SMTP server.

If you're getting "authentication required", you must first enter your user name and password and issue at least one request such as check for new mail. Even though SMTP does not require a username and password to SEND email, many SMTP servers still implement this by making you log in and check your mail via POP or IMAP before you can send any.