[java] Must issue a STARTTLS command first

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)