[sudonim]

Typekit gets you Proxima Nova as part of their package among others fonts. It's not bad, but for something like email, it may be helpful to use the standard stack of fonts.

My app sends emails (not a competitor). The interface uses proxima nova, but the email rendering is generally in an iframe with default sans-serif or serif font-faces.

If you use sans-serif as the first font-face that will default to Helvetica on Macs and Arial on Windows.