What does 'Unauthorized Error' mean?
Merchant Mail can only access your Shopify store data if you give it permission. This permission is granted when our app is first installed.
If you remove permission, which is normally done by uninstalling the app through the Shopify App Store, then Merchant Mail will return an "Unauthorized Error" since we can no longer access your Shopify data.
To fix this issue, you can reinstall the app. Once that's done, if you look at a different email message, or restart Gmail, then you should no longer see the error.