Earlier quoted context omitted.
You can self host forwardemail.net: https://github.com/forwardemail/forwardemail.net I haven't tried it myself but the maintainer is prominent in open source so I'd be pretty optimistic.
I'm planning to open-source the service, anyone will be able to host an instance and internal API to use it will be documented.
Totally fine if you don't know, but I may hold off investing time into something else if you expect it to be soon-ish.