Most people these days are using a service provider for SMTP in production, so it isn't really possible to keep your prod and test configs in sync.

I do agree about using testing everything real tools, which is why this (fakemail) uses OpenSMTPD as the mail server.

All the work is in configuring it (similar complexity to your postfix configuration by the looks of it), interfacing to it, and deploying it (currently using ansible but will probably dockerize it).

The fact that no emails can get through to a real recipient is a feature.