How does Cortana daily “Briefing” work?

You will receive a daily “Briefing” email. This email assembles existing information (i.e. tasks, documents, emails, and meetings) to which you already have access and summarizes them in an organized manner. You will receive a personalized brief email in Outlook at the beginning of each workday.   The email comes from cortana@microsoft.com.

What if I do not want to receive the "Briefing"?  

There is a unsubscribe link included within the daily briefing message itself at the bottom.   Alternately, you can choose to opt-in (or back out) at any time by visiting cortana.office.com (while logged into Office 365). Note: Any changes you make to the settings will require 24 hours to take effect.