Please could you tell me which programme for public key encryption one can use in eM Client? Is it GnuPG or the library that is used in the new Thunderbird versions? Or comes eM Client alresd- with a seperate implementation of the OpenPGP standard?
You don’t need any additional application to use encryption in eM Client.
You can import your friends public keys so you can send them encrypted messages, and you can import your own keys if you have generated them elsewhere. eM Client can generate them for you if you don’t already have them. eM Client also has a public key server, so you can upload your public keys so anybody can send you encrypted messages, or you can set eM Client to automatically attach your public key to all outgoing messages.