Mail proxy Nginx

Hi guys,

I would like to get some information about Nginx.
I have a project for my company and we would like to protect our mail server from external connection.
External user can only access to their mail with a VPN session (fortigate VPN).
We would like to avoid to let our mail server with a direct access from internet and would like to set up a proxy in front.

Is Nginx appropriate for that ? Would it work under the VPN connection from our external users ?

Thank you for your help.