Data.t is an on-disk Email.t. The type is abstract because we store a dot-encoded
email on disk.
val to_email : t ‑> Async_smtp_types.Email.tval of_email : Async_smtp_types.Email.t ‑> tval load : string ‑> t Async.Deferred.Or_error.tval save : ?temp_file:string ‑> t ‑> string ‑> unit Async.Deferred.Or_error.t