Which mail server is better?

Question

Email

I have a Web site on my own server. Like most sites, it has the ability to send emails to users' emails. Only now this is implemented through GMail, and now there is a need to move away from third-party Mail Servers and install your own independent Mail server on the hosting.
Requirements:
1 It is desirable that the server be compatible with both Windows and linux debian.
2 It is desirable that it be free.
So far, this is where the requirements end, since I have not yet encountered them and therefore do not know By what criteria they are still evaluated.
Tell me if there are ready-made solutions for the implementation of my task?

Answer 1

With free good Courier Mail Server

Answer 2

"Compatible" means "runs under" and you haven't chosen an infrastructure yet?
Good - Kerio Connect with a well-known potion. (or buy, if you paid for Gmail, then you won’t really notice)
Free almost everything is a mixture of different services + third-party web muzzles associated with them, but if there is no experience, such an assembly will be difficult to support.