Currently I have two options in "Account Settings" under "End-to-End Encryption" for sending messages:
o Disable encryption for new messages
o Enable encryption for new message
If I choose "Disable" I will have to remember, if the recipient might have a cert or key I already know or look it up. But usually I do forget encryption then completely and send unencrypted messages, even if it would be possible. If I choose "Enable", I will have to turn encryption off manually for every recipient I don't have a cert or key for.
Thunderbird knows about all keys and certs I already received, so it can select the best possible encryption itself.
I would like to recommend a third option for the settings:
o Encrypt, if possible
When selected, Thunderbird will use encryption for every message I write to recipients with certs or keys I already received and won't encrypt, when it's not possible.