Use E-mail Sensitivity header for SPX Encryption
Rather than require a plugin to encrypt an e-mail can you add the option of using the Sensitivity header which has been around for quite a long time now and is used by your competition (IronPort) to trigger encryption. A simple check box in the setup of the SPX profile will be sufficient. Just seems like you are trying to re-invent the wheel with your Outlook Add-in which in its current state on the UTM won't even install on most computers without an error.
Jeff B commented
Actually this functionality may already exist, under Email Protection, Content Scanning Rules you can create a rule that will encrypt an e-mail based on a header value.