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 following article:
Security in Office 365 White Paper
For a more comprehensive and detailed treatment of security in Office 365 download the following article:
Office 365 Security and Service Continuity Service Description
Ross Carter, CISSP (Office 2010 Security Resource Center on TechNet)