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.
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)