diff --git a/backend.go b/backend.go
index 9885d8e..13e7081 100644
--- a/backend.go
+++ b/backend.go
@@ -76,6 +76,8 @@ type Session interface {
 	// Add recipient for currently processed message.
 	Rcpt(to string) error
 	// Set currently processed message contents and send it.
+	// 
+	// r must be consumed before Data returns.
 	Data(r io.Reader) error
 }