I treat this as an insurance policy. Even in this thread people mentioned how Maddy, which is an alternative modern full stack email solution in a single binary, lacks development efforts.

This is why we have this fantastic release for Stalwart - free shit.

Also as of now enterprise is for $0.2 per account per month which is extremely cheap unless somebody wants to build a big spam farm, of which as civilized Internet user I don't support. Obviously this might change, but even if you can always built multi-tenancy layer by yourself if you really need it - rest of the codebase is AGPL.