Does anyone know how to safely serve a licensed font for use in an email?

fonts.com says it's ok to use their fonts in email by self-serving it, as long as you can prevent people from hotlinking to it. The only solution I've found to do this is to use the server to reject any requests that don't have the whitelisted domain in the document.referrer value. I don't think email clients will have this value set in a predictable way. Is it possible to do this some other way?

I've checked out this post:
http://stylecampaign.com/blog/2015/02/web-font-services-for-email/

Does anyone have experience using anyone of these services for using a licensed font in an email?