BSMTP writing
val write : ?skip_prelude_and_prologue:bool ‑> ?log:Async_smtp.Mail_log.t ‑> ?flows:Async_smtp.Mail_log.Flows.t ‑> ?component:Async_smtp.Mail_log.Component.t ‑> Async.Writer.t ‑> Async_smtp.Types.Envelope.t Async.Pipe.Reader.t ‑> unit Async.Deferred.Or_error.t