To encrypt an email, you will require the recipient's public key. As long as you have the correct key the lock symbol will turn Green (for iOS and Android) and Orange (for macOS) which indicates if an outgoing email is encrypted.
You may verify that your sent emails have been encrypted by opening your message in another mail client. You will see an attachment in body of your emails which are completely encrypted and unreadable. Your recipient can decrypt the email using their secret key (which corresponds to the public key used to encrypt the email).