Proxmox Mail Gateway is open-source (licensed under AGPLv3) - thus you can use it without any subscription.
the complete functionality is available in all released versions.
You need a subscription to:
* get access to the pmg-enterprise repository - where the software gets released last and is thus better tested
* get support via our enterprise portal (for subscriptions with level Basic or higher)
additionally you support the development of PMG with a subscription.