The email plugins for various clients make it easy to mistakenly think you’re sending an encrypted email.

Ok. Now I know what’s the issue but this is not the problem with gpg. Nah, gpg integration with thunderbird is so flawless that it clearly says when it’s encrypting when not. Also you can see the raw email content and then you see whether it’s plaintext or ciphertext. I’m using thunderbird with gpg very often so I know how it works nicely with gpg