Apps that can get direct access to your email via API, I'm assuming. Something like Boomerang, that can schedule emails to be sent at a later date for example. Not much different from Thunderbird or Outlook addons.
Though, you shouldn't use any Chrome plugins, unless you absolutely have to. They're not as policed as say the app store.
If you have to access your banking info for example, you can use ctrl+shift+n to open up an incognito window where all plugins are disabled by default. Also make sure you have 2FA.
This is a scare email with no real basis for concern. Yes, third parties might be able to scan your email, but you have to give them permission to do so. And even then is very rare that they do it. Honestly, it’s not really something that you should worry about.
"Developers may share data with third parties so long as they are transparent with the users about how they are using the data," Susan Molinari, VP of public policy and government affairs for the Americas at Google, said in the letter to Senators, which was obtained by CNNMoney.