smtpsrv: Only allow authenticated email on submission

The submission port is expected to be used only by authenticated
clients, so this patch makes chasquid enforce this, which also helps
to reduce spam.

https://www.rfc-editor.org/rfc/rfc6409.txt

internal/smtpsrv/server_test.go

@@ -146,6 +146,15 @@ func TestAuth(t *testing.T) {
 	sendEmailWithAuth(t, c, auth)
 }
 
+func TestSubmissionWithoutAuth(t *testing.T) {
+	c := mustDial(t, ModeSubmission, true)
+	defer c.Close()
+
+	if err := c.Mail("from@from"); err == nil {
+		t.Errorf("Mail not failed as expected")
+	}
+}
+
 func TestAuthOnSMTP(t *testing.T) {
 	c := mustDial(t, ModeSMTP, true)
 	defer c.Close()