ghostwheel42's suggestion

core/admin/mailu/internal/views/postfix.py

@@ -159,20 +159,13 @@ def postfix_sender_rate(sender):
 def postfix_sender_access(sender):
     """ Simply reject any sender that pretends to be from a local domain
     """
+    if '@' in email:
+        if email.startswith('<') and email.endswith('>'):
+            email = email[1:-1]
         try:
-        if not is_void_address(sender):
             localpart, domain_name = models.Email.resolve_domain(sender)
-            return flask.jsonify("REJECT") if models.Domain.query.get(domain_name) else flask.abort(404)
+            if models.Domain.query.get(domain_name):
+                return flask.jsonify("REJECT")
         except sqlalchemy.exc.StatementError:
             pass
     return flask.abort(404)
-
-
-def is_void_address(email):
-    '''True if the email is void (null) email address.
-    '''
-    if email.startswith('<') and email.endswith('>'):
-        email = email[1:-1]
-    # Some MTAs use things like '<MAILER-DAEMON>' instead of '<>'; so let's
-    # consider void any such thing.
-    return '@' not in email