Here are two examples to show you how to use JavaMail API method to send an email via Gmail SMTP server, using both TLS and SSL connection.

To run this example, you need two dependency libraries – javaee.jar and mail.jar, both are bundle in JavaEE SDK.

Outgoing Mail (SMTP) Server
requires TLS or SSL: smtp.gmail.com (use authentication)
Use Authentication: Yes
Port for TLS/STARTTLS: 587
Port for SSL: 465

GMail SMTP detail here – http://mail.google.com/support/bin/answer.py?hl=en&answer=13287

1. JavaMail – GMail via TLS

Send an Email via Gmail SMTP server using TLS connection.

Example updated on 19/03/2012
Thanks inputs from Sumeet Kumar Yadav and vihor. The below example is updated to get rid of the previous “javax.mail.AuthenticationFailedException” error. It working perfect now.
package com.mkyong.common;
 
import java.util.Properties;
 
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
 
public class SendMailTLS {
 
	public static void main(String[] args) {
 
		final String username = "username@gmail.com";
		final String password = "password";
 
		Properties props = new Properties();
		props.put("mail.smtp.auth", "true");
		props.put("mail.smtp.starttls.enable", "true");
		props.put("mail.smtp.host", "smtp.gmail.com");
		props.put("mail.smtp.port", "587");
 
		Session session = Session.getInstance(props,
		  new javax.mail.Authenticator() {
			protected PasswordAuthentication getPasswordAuthentication() {
				return new PasswordAuthentication(username, password);
			}
		  });
 
		try {
 
			Message message = new MimeMessage(session);
			message.setFrom(new InternetAddress("from-email@gmail.com"));
			message.setRecipients(Message.RecipientType.TO,
				InternetAddress.parse("to-email@gmail.com"));
			message.setSubject("Testing Subject");
			message.setText("Dear Mail Crawler,"
				+ "\n\n No spam to my email, please!");
 
			Transport.send(message);
 
			System.out.println("Done");
 
		} catch (MessagingException e) {
			throw new RuntimeException(e);
		}
	}
}

2. JavaMail – GMail via SSL

Send an Email via Gmail SMTP server using SSL connection.

package com.mkyong.common;
 
import java.util.Properties;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
 
public class SendMailSSL {
	public static void main(String[] args) {
		Properties props = new Properties();
		props.put("mail.smtp.host", "smtp.gmail.com");
		props.put("mail.smtp.socketFactory.port", "465");
		props.put("mail.smtp.socketFactory.class",
				"javax.net.ssl.SSLSocketFactory");
		props.put("mail.smtp.auth", "true");
		props.put("mail.smtp.port", "465");
 
		Session session = Session.getDefaultInstance(props,
			new javax.mail.Authenticator() {
				protected PasswordAuthentication getPasswordAuthentication() {
					return new PasswordAuthentication("username","password");
				}
			});
 
		try {
 
			Message message = new MimeMessage(session);
			message.setFrom(new InternetAddress("from@no-spam.com"));
			message.setRecipients(Message.RecipientType.TO,
					InternetAddress.parse("to@no-spam.com"));
			message.setSubject("Testing Subject");
			message.setText("Dear Mail Crawler," +
					"\n\n No spam to my email, please!");
 
			Transport.send(message);
 
			System.out.println("Done");
 
		} catch (MessagingException e) {
			throw new RuntimeException(e);
		}
	}
}
java.net.UnknownHostException: smtp.gmail.com
Some hit the UnknownHostException: smtp.gmail.com, try ping smtp.gmail.com and make sure you got a response (able to access). Often times, your connection may block by your firewall or proxy behind.
You might also like this tutorials :
Hibernate Tutorials
Maven Tutorials
Apache Wicket Tutorials
JAX-RS Tutorials
Spring Tutorials