[lhaussknecht]

There is an native U2F implementation for Android by yubico: https://github.com/Yubico/android-u2f-demo. Also take a look at this webinar https://www.yubico.com/2015/10/u2f-webinar-from-concept-to-i.... The client data part is what you have to send to the authenticator.

[Freak_NL]

The client-side code and the standard are fairly trivial to implement — thanks to Yubico as well — but the problem lies in designing and standardising an extension to the existing mail protocols, and getting that accepted as a common feature for e-mail providers (such as Google and Microsoft) and a common feature in MTAs and MDAs.