Java Mailing Logic: Could not convert socket to TLS

In one application, I implemented mail sending logic using java. I used smtp.gmail.com over 587 port with a valid gmail id and password. In development environment everything is working fine. But in production environment I need to use a different mailing server say smtp.xyz.in over port 25 with a valid email id and password on that domain.

When I continue with SSL enable with following code:

I am getting an error

Could not convert socket to TLS

SunCertPathBuilderException: Unable To Find Valid Certification Path To Requested Target

=======================================================

final ResourceBundle rsbd=ResourceBundle.getBundle("main/ResourceBundle/Dyna");

        // -- Attaching to default Session, or we could start a new one                                            

                    props.put("mail.smtp.host", smtpServer);

                props.put("mail.smtp.auth", "true");

                props.put("mail.debug", "true");

            props.put("mail.smtp.port", port); 

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

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

  Session session =Session.getInstance(props, new Authenticator() {protected PasswordAuthentication getPasswordAuthentication() {return new PasswordAuthentication(admin_mail, admin_password);}});

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

     // -- Set the FROM and TO fields --
      msg.setFrom(new InternetAddress(from));

      msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse(email, false));

      msg.setSubject(subject);
      msg.setText(emailContent);

      // -- Set some other header information --
      msg.setHeader("X-Mailer", "LOTONtechEmail");
      msg.setSentDate(new Date());

        // -- Send the message --
        Transport.send(msg);

When I am removing EnableSSL and trying by adding the following code:

(getting javax.mail.AuthenticationFailedException:535 5.7.3 Authentication unsuccessful)

==========================================================================

props.put("mail.smtp.socketFactory.port","25");

  props.put("mail.smtp.socketFactory.class", "javax.net.ssl.SSLSocketFactory");

  props.put("mail.smtp.socketFactory.fallback", "true");


  MailSSLSocketFactory sf=new MailSSLSocketFactory();

  sf.setTrustAllHosts(true);

  props.put("mail.smtp.ssl.socketFactory", sf);

By googling enough in last 3 days, I understand that I need to configure for trusted certificate like given here.

But I want to continue without encryption and without mugging to enable SSL. Is there a way to send emails by java programs through our own domain without enabling SSL. Any help will be appreciated.