The roadmap is actively in development and it will be open-source before it goes live to a small subset of early access users. So the intention is NOT to create

The roadmap is actively in development and it will be open-source before it goes live to a small subset of early access users. So the intention is NOT to create plugins for mail clients. There has been a lot of advancement in encryption since PGP (GPG being an implementation of PGP) and the idea right now is to implement a double-ratchet algorithm for v1.0, which has the advantage of forward secrecy. Also, yes, Tildamail will interoperate with normal everyday SMTP emails. Thanks!