Systems and methods are described for communication of a third-party application server with a third-party email client plug-in. The systems and the methods enable the third-party application server to provide a plug-in header contained in a message. The plug-in header may include an X-header. The X-header may be injected into the message. The plug-in installed within an email client receives the message. The plug-in is configured to process the plug-in header to identify one or more instructions to perform an action of one or more actions. The one or more instructions may relate to a property of a user and/or a property of the email client of the user. Responsive to the one or more instructions, the plug-in performs the action.