My mail server was sending plenty of dummy emails using a user mail account, most of them were stuck in my mail queue due to blacklist/ graylist failure at first i tried to change the user's mail password to more complex one, but the issue still exists, and the user confirms he never used his email for sending any mails,
I also tried to use some php script to send email using my account without SMTP authentication, and it failed,
>SMTP -> get_lines(): $data is "554 5.7.1 <firstname.lastname@example.org>: Relay access denied
So how can someone use my mail server to do so?!