// `arg` (the `MessageArg` input resolver) is used by the protocol
// send/save commands too, so it stays compiled. `compose`/`send` (and
// their `builder`/`handler` helpers, plus the `cli` enum that hosts
// them) also work over SMTP, so they carry `any(backend, feature =
// "smtp")`; the remaining commands need a mailbox backend, so they carry
// `backend`.