Unrecognized SSL message, plaintext connection? Exception

I think this is due to the connection established with the client machine is not secure.

It is due to the fact that you are talking to an HTTP server, not an HTTPS server. Probably you didn't use the correct port number for HTTPS.

javax.net.ssl.SSLException: Unrecognized SSL message, plaintext connection?

You should have a local SMTP domain name that will contact the mail server and establishes a new connection as well you should change the SSL property in your programming below

javax.net.ssl.SSLException: Unrecognized SSL message, plaintext connection

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