Recently i have encountered a weird problem with my virtualmin server. Email suddenly stops working. I tried sending test emails but they were not delivered. Nothing is found in mail.log. The last line in mail.log was a few days ago. I checked postfix status and see postfix was running normally. Then i reboot the entire server, IT WORKS.
So now the problem has been resolved, but i don't know what is the cause of the problem, what i should do to prevent it from happening again, and what i should do to debug if it happens again in the future