Text- in a similar way TextSecure did it, would be my guess. They have something called Silent Text, and they're using the ideas here I believe:

http://eprint.iacr.org/2014/036.pdf

Email? They've announced the DarkMail protocol last year, and should be coming soon:

http://darkmail.info/

https://www.youtube.com/watch?v=IgV_Z6V_llk