Microsoft Office 365 provides enterprise-grade security for businesses of all sizes. To learn more about the security practices and technologies in Office 365 see the following articles available on the Microsoft Download Center.
For an overview of security in Office 365 download the article entitled "Security in Office 365 White Paper."
And, for a more comprehensive and detailed treatment of security in Office 365 download the following article linked under.
"Office 365 is a suite of Internet-based services that are designed to help meet your needs for robust security, 24/7 reliability, and user productivity."
"This set of documents provides service descriptions for the components of the suite," noted Microsoft.