Encryption mandatory needed features to work properly

Is there a way to enable and disable encryption on a per message basis. I also want to to reply to unencrypted messages without encryption and to the encrypted messages with encryption automatically. As opposed to just all encrypted or all not. Not all of my emails should be encrypted I am afraid at the moment that i would reply to an encrypted email without encryption so i just ended up completly removing the cert.

Also is there a way to make the fact that a message is encrypted clearer