S/MIME Sign box checked by default

Content

I posted this in the Agent Desktop forum too, but it seemed like an Add-In is the most likely solution. Has anyone built an Add-In so that all outbound emails have a digital signature? Does it work for automated responses? Any suggestions or lessons learned would be very appreciated!

----

Our client is requiring that all outgoing emails be digitally signed. We already have certificates in place for our OSC mailboxes, but I don't want our agents to have to check the Sign box each time they send an email. 

Is anyone aware of a solution for having the Sign box checked by default or for the signature to be added automatically through some other means? I'm hoping there is an add-on for the admin layer, rather than having to send the emails through an API.