Rated 5 out of 5 stars

This works pretty well!
Also note the option to enable both signature and encryption by entering "sign;encrypt".

But I have a feature request:
Sending a mail to some recipients having "encrypt" set an some that don’t have it set (because you don’t have a certificate from them) fails with an “certificate missing” error message.
The user should be asked if the message should be sent unencrypted in that case.

This review is for a previous version of the add-on (1.0). 

Thanks for your comment and hint, so I've added in version 1.1 an additional check for recipient certificates before setting encryption (alert on missing ones can be disabled in options)