Email does work fine. We have a similar solution for encryption: PGP. If you do the encryption yourself instead of relying on your email client to handle it, it works fine. Type your text, encrypt it, then put it in an email; get a response, decrypt it, and read it — it's easy, and it works. People who say PGP is too hard to use are usually relying on a plugin for their email client, or some other unreliable, "encryption magic" tool that tries to hide how PGP works behind an incompatible abstraction.

Simple tools that do one thing well are almost always best.