diff --git a/tools/linter_lib/custom_check.py b/tools/linter_lib/custom_check.py
index 448cd277c2..5624cc5aad 100644
--- a/tools/linter_lib/custom_check.py
+++ b/tools/linter_lib/custom_check.py
@@ -362,7 +362,7 @@ def build_custom_checkers(by_lang):
     ]) + whitespace_rules + comma_whitespace_rule
     python_rules = cast(RuleList, [
         {'pattern': 'subject|SUBJECT',
-         'exclude_pattern': 'subject to the|email',
+         'exclude_pattern': 'subject to the|email|outbox',
          'description': 'avoid subject as a var',
          'good_lines': ['topic_name'],
          'bad_lines': ['subject="foo"', ' MAX_SUBJECT_LEN'],