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)

Rated 5 out of 5 stars

Yeah this is kind of cool and does what it states very well!

Only thing left is to do it the other way round*: Usually I have "digitally sign by default" activated and want it to deactivate this for certain contacts. So a good addition would be to add the switch no-sign and possibly no-encryt.

Keep up the good work!

*[EDIT] nosign and noencrypt is working like a charm in the new version. Thanks! :)

This review is for a previous version of the add-on (0.1.2). 

Nice idea, no-sign and no-encryt, I'll do that in the next version.

PS: sent you an e-mail.