There is not much to document on how to set it up. The gui is very intuitive.

You need to setup the relay domains, transports and Options>DNSBL under configuration > mail proxy

It depends a bit on your needs. My proxmox setup is like multiple nodes (computers) with local (2 drives with ZFS mirrorig), they all use a truenas server as NFS host for data storage. For some things I use conaitners (LXC) but other thing I use VMs.

It is, I looked at several vendor’s and it’s all te same except for the nice gui. They all have their own blacklist that they feed with the spam/ham queries from their devices.

They don’t have to cost money. The mail filter appliances are all based on postfix, spamassassin and a virus scanner like clamav. The thing you pay for is the nice gui.

I recently moved to Mailcow, it’s a one in all solution. My spam filter is Proxmox mail gateway, also very user friendly.

I also have a mail filter, I have been hosting my own mail server for the last 25 years.

Yes, that works just fine. Wen my daughter was at her boyfriend’s place she used a wireguard tunnel to my house to use Netflix.