11.11. How do I automatically encrypt my email messages?

Select Encrypt messages by default in your account settings. Then enable OpenPGP → Preferences → Key Selection → No manual key selection. This way, outgoing messages are encrypted automatically if a public key is available.

This method of default encryption works only for trusted public keys. So either you enable OpenPGP → Preferences → Always trust people's keys (which is slightly insecure), or you have to check every public key and sign it to make the key trustworthy (in this case you should only sign them locally, non-exportable).

There is one problem with this method: you never know if a mail will be encrypted when you just push the Send Now button. The workaround for this is to click on Send Later and look in your Unsent Messages folder.