This window is a normal step of Gmail accounts set up, as can be seen in our step-by-step Gmail/G Suite setup on our website.

It's part of Google's authentication called oAuth, where instead of giving the external application (eM Client) your password, you just give access from your Google account settings.

So, log in with your Gmail account and allow access for eM Client by clicking on "Accept" on the second browser page right after you log in to the browser.

eM Client will be granted an authentication token that you can revoke at any time using your online Google accounts settings.

Read more in our blog: OAuth authentication with Gmail