Putting main website and mail on seperate servers using single domain

[andy]

It really looks complicated for me. lol

If I understand correctly, you can do it in a simple way:

your requirements:
main website -> window server (existing)
mail -> handled by a mail server on linux (not existed now and you want to build it on sentora)
Domain Name Server (existing somewhere)

To achieve the above, you need the following:
1) create a linux instance
2) install sentora on it
3) setup all the necessary protection (virus scan, spam control, SSL, authentication, reverse IP, etc)
4) create a virtual domain on sentora for your company and setup the email accounts 
5) once the above 4 are done, change your domain name server MX records to point to this mail server

Good luck,

Andy