To verify from your mail logs if the message is indeed leaving the system, go to Webmin >> System >> System Logs and then View mail.log
Sending test messages to a Gmail account is not the best way to test a mail server because Gmail maintains the highest of standards in the industry with regard to the email it permits to land in a user’s inbox. If your server has not been configured correctly, complete with RDNS and the domain with DKIM & SPF, then email might not be delivered at all: neither to Gmail’s inbox nor to any other the folders like Promotion or Spam. Maybe that’s what’s happening with your system @ihabhamed.
I suggest you use a (free for limited use) tool like https://www.mail-tester.com/ and keep tweaking till you get 10/10 before you test with Gmail.