//! # Shared API
//!
//! The cross-protocol commands, behaving the same whatever backend serves
//! the active account, and the client dispatching them onto it.
//!
//! The `backend` cfg build.rs sets is on when a mailbox backend is
//! compiled in. Most commands need one, but composing and sending also
//! work over SMTP alone, so those carry `any(backend, feature = "smtp")`.