Must issue a STARTTLS command first

You are probably attempting to use Gmail's servers on port 25 to deliver mail to a third party over an unauthenticated connection. Gmail doesn't let you do this, because then anybody could use Gmail's servers to send mail to anybody else. This is called an open relay and was a common enabler of spam in the early days. Open relays are no longer acceptable on the Internet.

You will need to ask your SMTP client to connect to Gmail using an authenticated connection, probably on port 587.

smtp port and socketFactory has to be change

    String to = "[email protected]";
    String subject = "subject"
    String msg ="email text...."
    final String from ="[email protected]"
    final  String password ="senderPassword"


    Properties props = new Properties();  
    props.setProperty("mail.transport.protocol", "smtp");     
    props.setProperty("mail.host", "smtp.gmail.com");  
    props.put("mail.smtp.auth", "true");  
    props.put("mail.smtp.port", "465");  
    props.put("mail.debug", "true");  
    props.put("mail.smtp.socketFactory.port", "465");  
    props.put("mail.smtp.socketFactory.class","javax.net.ssl.SSLSocketFactory");  
    props.put("mail.smtp.socketFactory.fallback", "false");  
    Session session = Session.getDefaultInstance(props,  
    new javax.mail.Authenticator() {
       protected PasswordAuthentication getPasswordAuthentication() {  
       return new PasswordAuthentication(from,password);  
   }  
   });  

   //session.setDebug(true);  
   Transport transport = session.getTransport();  
   InternetAddress addressFrom = new InternetAddress(from);  

   MimeMessage message = new MimeMessage(session);  
   message.setSender(addressFrom);  
   message.setSubject(subject);  
   message.setContent(msg, "text/plain");  
   message.addRecipient(Message.RecipientType.TO, new InternetAddress(to));  

   transport.connect();  
   Transport.send(message);  
   transport.close();
   }  

hope it will work for you..

Adding

props.put("mail.smtp.starttls.enable", "true");

solved my problem ;)

My problem was :

com.sun.mail.smtp.SMTPSendFailedException: 530 5.7.0 Must issue a STARTTLS command first. u186sm7971862pfu.82 - gsmtp

at com.sun.mail.smtp.SMTPTransport.issueSendCommand(SMTPTransport.java:2108)
at com.sun.mail.smtp.SMTPTransport.mailFrom(SMTPTransport.java:1609)
at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1117)
at javax.mail.Transport.send0(Transport.java:195)
at javax.mail.Transport.send(Transport.java:124)
at com.example.sendmail.SendEmailExample2.main(SendEmailExample2.java:53)