Thank you for this useful add-on. It fills a fundamental gap in Thunderbird's functionality. However, I'd like to suggest an additional feature (or maybe a separate add-on): Can you make Thunderbird optionally ask me if I want to sign my e-mail? I have a smart card with my certificate on it. For less important mails I don't need it and don't take it out of my pocket. Therefore I have mandatory signing turned off. But when it comes to writing official mails, I often forget to sign and would really appreciate Thunderbird reminding me.
This review is for a previous version of the add-on (1.0).