Can't connect to SMTP server with Java

问题:

I want to send an email from Java using the Java Mail API (javax.mail.*). I can access the SMTP-server via Thunderbird with the following settings:

• Server: math.uni-freiburg.de
• Port: 465
• Username: MY_USERNAME
• Authentification: password, normal
• Security: SSL/TLS

I know nothing else about the SMTP server.

With my code I always get the error: Could not connect to SMTP host: math.uni-freiburg.de, port: 465, response: -1

String SMTP_HOST = "math.uni-freiburg.de"
String SMTP_USER = "[e-mail]";
String SMTP_PASSWORD = "MY_PASSWORD";
String SMTP_PORT = "465";

final Properties props = new Properties();
props.put("mail.smtp.host", SMTP_HOST);
props.put("mail.smtp.port", SMTP_PORT);
props.put("mail.transport.protocol","smtp");
props.put("mail.smtp.auth", "true");
//props.put("mail.ssl.enable", "true");
props.put("mail.smtp.starttls.enable", "true");
props.put("mail.smtp.tls", "true");
props.put("mail.smtp.ssl.checkserveridentity", "true");

final javax.mail.Authenticator auth = new javax.mail.Authenticator() {
   @Override
   public PasswordAuthentication getPasswordAuthentication() {
      return new PasswordAuthentication(SMTP_USER, SMTP_PASSWORD);
   }
};

Session session = Session.getInstance(props, auth);


Message msg = new MimeMessage(session);
msg.setFrom(new InternetAddress("[e-mail]", "MY NAME"));
msg.addRecipient(Message.RecipientType.TO, new InternetAddress("[e-mail]", "asdf asdf"));
msg.setSubject("SUBJECT");
msg.setText("THE MESSAGE");
msg.saveChanges();
Transport.send(msg); 

I already tried to change some of the properties but since I don't really know what they do this wasn't very successfull.

Any ideas what I could change?