Exchange 

This is the engine behind Outlook. It supplies you with email, calendar, contacts, tasks, journaling, and notes capabilities. This portion of Office 365 gives an enterprise-grade, professional email with 50 GB of space using your company's custom domain. You also get the ability to share calendars with others in your organization. Exchange is included with the Business Essentials, Business Premium, E1, E3, and E5 plans. If you have an E3 or E5 plan, you will get a 100 GB mailbox with unlimited archiving.