[Repeat rant about people not adopting PGP when we've had it for 27 years]
Nothing about a smartphone prevents email from being encrypted. The fact that nobody's sending encrypted email does, but that's a user adoption problem rather than a technical problem. The technical problem is solved, solved well, and has been solved for decades.
Key management is moderately difficult, and more of a UX problem than a technical one. A UX very much like that of iMessage could be built on top of PGP and keyservers.
We've had the ability to do this for a long time, but only a few major players are in a position to ensure sufficient user adoption, and they're not interested in creating anything that doesn't drive users to their walled gardens.
The bigger problem is that neither Apple nor Google have implemented device-local PGP encryption in their default email clients. Apple is one-step ahead and actually has had s/mime support for a while, but it really needs wide support on Android phones as well to become ubiquitous. It would set an implied standard for all other email apps on iOS and Android.
Nothing about a smartphone prevents email from being encrypted. The fact that nobody's sending encrypted email does, but that's a user adoption problem rather than a technical problem. The technical problem is solved, solved well, and has been solved for decades.