Add status callback into a Session's Data interface

backend.go

@@ -79,7 +79,7 @@ type Session interface {
 	// Set currently processed message contents and send it.
 	//
 	// r must be consumed before Data returns.
-	Data(r io.Reader) error
+	Data(r io.Reader, statusCb func(status *smtp.SMTPError)) error
 }
 
 // LMTPSession is an add-on interface for Session. It can be implemented by