Install the PK4Connect app in your Salesforce org. The first time that you click the Send Email Blast, for sending emails, you will be asked the authorize PK4Connect - you will see a green button asking you to Process OAuth. Once you click this button, you will have authorized PK4Connect to pull data from email servers and update your Email Campaign statistics.
In your Production org, you will need to process the OAuth only once, when you install PK4Connect. After that you need to do this step only if your OAuth token has expired.
In your Sandbox orgs though, OAuth is not as reliable. Your OAuth token expires at random times. So you may need to Authorize the app multiple times in the Sandbox.
To process the oAuth after the initial install, go to App Launcher and find the PK4Connect OAuth (Admin only) tile and click it. Click the Allow button on the Allow Access screen, once the app is authorized, you will see a Success screen. Now the PK4Connect app is ready for use.
Please note that you will NOT need to do the OAuth process every time that you send out an email blast. You will need to do it once initially after you install PK4Connect and later only if your OAuth token has expired.