@Rick_C137 They probably use “Memory Hole” (https://github.com/autocrypt/memoryhole/blob/master/specs/draft-memoryhole.md), that’s what Enigmail did before.

I’m not entirely sure what you hope to achieve: have a GPG encrypted subject, and have ThunderBird automatically understand that it’s encrypted, so it can be automatically decrypted?

Since you’re saying you’re building software to support this, what are you building? A ThunderBird plugin that can do this? Or just standalone software that you want to make compatible with ThunderBird default way of handling encryption?