1. In your browser, sign in to your Google/Gmail account.
  2. Click the gear icon in the upper right-hand corner and select Settings:

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  3. Click the Labels tab, and under System label find All Mail and click “show”:

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  4. “All Mail” is showing up at the left of your Gmail screen.

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NOTE: If you want to know why what is difference between Inbox and All Mail, please check the following support note: What is Gmail “All Mail” – and what is difference between Inbox and All Mail.