Raspberry PI

The cost of doing business means you should buy a license for each user that needs an account.

https://www.linode.com/community/questions/22605/can-i-set-up-a-smtp-relay-server-on-linode

I have mine setup on a Supermicro itx-based machine with an Intel n3710 and 8G of RAM. It has four Intel-based ports but you can easily get away with two.

I used to run a Zotac ci323 with dual Realtek nics. Works fine for 300M up/down.

You’ll want two ports, one for WAN one for LAN and most nics will support VLANs if you need more than that. Any VPN or encryption will increase your cpu requirements. If your needs are low a cheap dual-nic Nuc like device works great.

depending on your use case these are cheap and cheerful.

https://www.newegg.com/sapphire-radeon-rx-6400-11315-01-20g/p/N82E16814202416