TO BE DONE
- implement DKIM when send out
- check if DKIM in the header are valid
https://stackoverflow.com/questions/13833098/how-to-sign-javamail-with-dkim
https://github.com/usrflo/DKIM-for-JavaMail
https://stackoverflow.com/questions/36149094/how-to-set-dkim-and-spf-in-exchanbge-mail-server?rq=1