Reject null senders delivering directly to mailing list

Bounces should use the -bounce address.

Mailman/Queue/LMTPRunner.py

@@ -468,6 +468,10 @@ class LMTPRunner(Runner, smtpd.SMTPServer):
                         ))
                     queue = Switchboard(mm_cfg.INQUEUE_DIR)
                 elif subaddress is None:
+                    # Reject null senders when delivering to the main list queue.
+                    if mailfrom == '<>':
+                        status.append('550 '+EERR_572)
+                        continue
                     msgdata['tolist'] = True
                     queue = Switchboard(mm_cfg.INQUEUE_DIR)
                 elif subaddress == 'request':