Your company subscribes to the Office 365 Enterprise El plan. You are the Microsoft Exchange Online administrator.
In the last week, employees have reported that they are receiving non-delivery report messages from other companies. The employees have not sent messages to
recipients within those companies.
You need to prevent non-delivery report messages from reaching employees.
Which content filter option should you turn on in the Exchange admin center?

A.
NDR backscatter

B.
Conditional Sender ID filtering: hard fail

C.
SPF record: hard fail

D.
Block all bulk email messages

Explanation:
http://blogs.technet.com/b/exchange/archive/2014/08/18/spam-email-and-office-365-environment-connectionand-content-filtering-in-eop.aspx