The plugin description suggests turning off all encryption in account settings and let "Encrypt, if possible" do the work. This is against "secure by default" and dangerous. I have used this setting, resulting in messages being sent unencrypted randomly. This happened after sending multiple encrypted emails in both ways. Using Thunderbird 52.7.0 (64-bit) and depending on file attachments, new email vs. responding, the emails have been sent unencrypted randomly. Of course Thunderbird had been restarted, no other plugins enabled (e.g. no Enigmail). I highly recommend to chose "Always encrypt" / "Force Encryption" in account settings. You will be asked to disable encryption on demand (e.g. if no certificate is available for the recipient), instead of sending unencrypted emails unintentionally because the plugin failed to enable encryption - for whatever reason. This seems to be a much safer approach! I am very disappointed some of my emails have been sent unencrypted by using the suggested settings.