Accept and handle emails sent with an empty 821.From / return-path as… (#561)

* Accept and handle emails sent with an empty 821.From / return-path as it would any other email.

pkg/server/smtp/handler.go

@@ -411,18 +411,6 @@ func (s *Session) parseMailFromCmd(arg string) {
 	from := m[1]
 	s.logger.Debug().Msgf("Mail sender is %v", from)
 
-	// Parse from address.
-	_, domain, err := policy.ParseEmailAddress(from)
-	s.logger.Debug().Msgf("Origin domain is %v", domain)
-	if from != "" && err != nil {
-		s.send("501 Bad sender address syntax")
-		s.logger.Warn().Msgf("Bad address as MAIL arg: %q, %s", from, err)
-		return
-	}
-	if from == "" {
-		from = "unspecified"
-	}
-
 	// Parse ESMTP parameters.
 	if m[2] != "" {
 		// Here the client may put BODY=8BITMIME, but Inbucket already
@@ -480,7 +468,7 @@ func (s *Session) parseMailFromCmd(arg string) {
 	// Ignore ShouldAccept if extensions explicitly allowed this From.
 	if extAction == event.ActionDefer && !s.from.ShouldAccept() {
 		s.send("501 Unauthorized domain")
-		s.logger.Warn().Msgf("Bad domain sender %s", domain)
+		s.logger.Warn().Msgf("Bad domain sender %s", origin.Domain)
 		return
 	}
 

pkg/server/smtp/handler_test.go

@@ -78,6 +78,11 @@ func TestGreetState(t *testing.T) {
 	}
 }
 
+// Messages sent with a null reverse-path are unusual,
+// but valid. They are used for delivery status
+// notifications, and also for some sorts of auto-responder
+// as part of bounce storm mitigation.
+// Sections 3.6.3 and  4.5.5 of RFC 5321 discuss them.
 func TestEmptyEnvelope(t *testing.T) {
 	ds := test.NewStore()
 	server := setupSMTPServer(ds, extension.NewHost())
@@ -85,14 +90,14 @@ func TestEmptyEnvelope(t *testing.T) {
 	// Test out some empty envelope without blanks
 	script := []scriptStep{
 		{"HELO localhost", 250},
-		{"MAIL FROM:<>", 501},
+		{"MAIL FROM:<>", 250},
 	}
 	playSession(t, server, script)
 
 	// Test out some empty envelope with blanks
 	script = []scriptStep{
 		{"HELO localhost", 250},
-		{"MAIL FROM: <>", 501},
+		{"MAIL FROM: <>", 250},
 	}
 	playSession(t, server, script)
 }