That would be whatever the heck is the standard XenForo bits are defined. Should be the "contact" email template, with a mess of phrases related to it.
The phrase is contact_email_body_html apparently. And showing {ip} is its master value. Is that an issue with XenForo or does this add-on tamper with that variable or something? I can't remember the last time I got a Contact Us email. I just encountered this a couple days ago while testing stuff.
Oh well, I'll leave that be for now I guess. Or I can just remove the IP section from that phrase. The content of the email have the IP address anyway.