About this Add-on
If "Custom1" contains either "sign" or "encrypt" or both, the corresponding message security option is set. The setting is done when saving or sending the message.
You can also disable signing or encrypting for a contact by setting "Custom1" to either "nosign" or "noencrypt" or both (useful when signing or encrypting by default and you want to exclude only some contacts).
Note: this add-on relies on Thunberbird's built in encryption and signing functionality. You have to set this up first before using this add-on, see http://kb.mozillazine.org/Message_security for details.