Hi Jianchun,
This kind of notification emails is sent from the SharePoint Online service by default. Based on my understanding, after the SharePoint Online Newsfeed service is provisioned completely for a user, the above email with the subject “Getting the most out of SharePoint” will be sent to this user automatically. If it is a non-admin user, the email notification will also be sent to the administrator to notify the administrator that there’s a user who is ready to use Newsfeed.
Furthermore, some other email notifications associated with the Newsfeed Settings will be also sent to the user. To avoid the email notifications of Newsfeed Settings, you can refer to the steps below:
1. Sign in to Office 365, click the down arrow on the user’s display name in the upper right corner and click About Me.
2. Click edit to edit your profile.
3. On the Edit Details page, click the ellipse (…), and click Newsfeed Settings.
4. By default, all the notifications are selected in the section of Email Notifications. You can unselect them so the email notifications will not be sent to you.
5. Click Save all and close.
Meanwhile, I invite you to submit comments about this to our product team so they can look into this feature. Your time and efforts are highly appreciated.
Regards,
Jerry Cen