I am working on routeing mail through tailscale to a relay, since my host, for whatever reason, blocks mail ports and charges to have them turned on.

Should work fine. Your provider can't stop you from opening ports unless its a shared environment and you don't have permission/the port is already in use. Generally what they do is just block connections via a router/firewall. So if you use a VPN you're sidestepping that issue. With the VPN in place, and the server online and running you should be able to connect via {VPN_IP}:995, etc.