How to find your email in Category Tabs.
Gmail is set up to sort your incoming email into different Category Tabs.
This feature can make it difficult to find your Mailing List email.
Tab categories can include:
Click the various Tabs to see emails in each category.
Mailing List messages often appear in either Forums or Promotions tab.
Gmail categorizes email messages automatically based on their algorithms and your past actions.
To learn how to change this setting, please see Gmail Help - Add or remove inbox categories & tabs in Gmail