In addition, I'd like to add that you can install/run PMG on top of standard Debian by using the
proxmox-mailgateway-container meta package (despite the name it doesn't have to be installed on a container).
So if for some reason the PMG ISO shouldn't support your FC connection, which is difficult to believe, you could simply use plain Debian with your running FC connection as base.
https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#pmg_install_on_debian_container