Nevermind, I found it.
Mail Policies > Text Resources > Add Text Resource
Set the type as Encryption Notification Template (HTML), edit the HTML and then also add one with a type Encryption Notification Template (text), edit the text. Then go into the Security Services > IronPort E-Mail Encryption > modify the Encryption profile and choose the new Text Resources created above.
Under Text Resources I was also able to create custom bounce messages with variables that display all the relevant information regarding the bounce.