Java mail API works on localhost bu not on google app engine - google-app-engine

Question

I have a sending email logic using Java Mail API and it works fine when I am on localhost but once deployed on google cloud platform, the email goes to my servlet but never gets delivered.
I bought the email from GoDaddy so it's: xxx#mydomain.com.
After reading docs on google cloud platform and some comments here on StackOverflow, I have configured firewall rules but nothing seems to work to allow ingress and egress on port 25, 465 and 587 (I know google doesn't allow traffic on port 25).
I don't wanna use 3rd party email senders like sendbird... because I was using elastic before and I didn't need a 3rd party email sender, JavaMail was enough.
So I think Java mail should be enough for GCP.
Can anyone help me out?
Here is my sending email logic
import java.io.UnsupportedEncodingException;
import java.util.Properties;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
/**
*
* #author sidibe ibrahim
* Sending email logic
*/
public class EmailSender {
static MessagingException me;
public static boolean sendMail(String from, String password, String message, String to[], String title) throws UnsupportedEncodingException {
String host = "smtpout.secureserver.net";
Properties props = System.getProperties();
props.put("mail.smtp.starttls.enable", "true");
props.put("mail.smtp.host", host);
props.put("mail.smtp.user", from);
props.put("mail.smtp.password", password);
props.put("mail.smtp.host", 465);
props.put("mail.smtp.auth", "true");
props.put("mail.smtp.ssl.trust", "*");
Session session = Session.getDefaultInstance(props, null);
MimeMessage mimeMessage = new MimeMessage(session);
try {
mimeMessage.setFrom(new InternetAddress(from, "xxx"));
InternetAddress[] toAddress = new InternetAddress[to.length];
for (int i = 0; i < to.length; i++) {
toAddress[i] = new InternetAddress(to[i]);
}
for (int i = 0; i < toAddress.length; i++) {
mimeMessage.addRecipient(Message.RecipientType.TO, toAddress[i]);
}
//sdd subject
mimeMessage.setSubject(title);
//set message to mimeMessage
mimeMessage.setText(message, "UTF-8", "html");
Transport transport = session.getTransport("smtp");
transport.connect(host, from, password);
transport.sendMessage(mimeMessage, mimeMessage.getAllRecipients());
transport.close();
return true;
} catch (MessagingException m) {
me.printStackTrace();
}
return false;
}
}

Answer 1

The App Engine Mail API (which also supports JavaMail) has already been deprecated.
Instead, GCP recommends to use a third-party mail provider such as:
SendGrid
Mailgun
Mailjet
EDIT
If you would still like to continue with the outdated solution, however, see the article on sending emails with the Mail API.