What materials (courses, books, articles, etc.) did you use to organize the security of mail services?

Question

Email

Good afternoon.
Currently using mdaemon 16.5.2 and mail service management and security is one of my downsides.
The task is to give access "outside" bypassing VPN channels, and this worries me. I
would like to plunge into the security of the service "from A to Z", but either I was looking badly, or "hands with eyes" ...
But somehow I did not find a selection of materials for study, so that you could advise?
Thank you.

Answer 1

The fact is that security is a whole complex of measures. Which spreads and affects systems and infrastructure AT ALL. Therefore, in the general case, no one solves the problem in the formulation of "ensure the security of the mail server." Because it doesn't make sense. Accordingly, no one writes such literature. Maximum - separate articles in separate blogs, on the topic "tighten some nuts in a particular software."