There is, however, an app like DeltaChat that looks more or less like WhatsApp, uses AutoCrypt for E2E encryption, and uses IMAP/SMTP as its storage and transport media.
Fun fact, the main reason I developed rpgp in the first place was to power the pgp portion pf deltachat, as we ran into the same problem of not having a good library based implementation. And spawning gnupg on a phone was never an option.