About this Add-on
This add-on sets S/MIME message security option "encrypt message", if valid X509 certificates are found for all of the recipients and for the sender himself. If these criteria are not matched, the option is not set.
You can disable confirmation message in the extension's options.
Change the default security/encryption setting in your account settings from "Required" to "Never (do not use encryption)" to let this add-on decide and encrypt only if possible.
For the list of available certificates see menu Tools -> Options -> Advanced -> Certificates -> View Certificates -> People.
Note: this add-on relies on Thunberbird's built in encryption functionality. You have to set it up first before using this add-on, see http://kb.mozillazine.org/Message_security for details.