The following page can be used to filter and compare features of Office 365 plans, Office 365 services, and on-premises products, such as comparing every feature of Exchange Server 2013 on-premises vs. Exchange online: http://technet.microsoft.com/en-us/office365/fp123607
This is extremely useful when exploring Exchange Online (and any Office 365 service for that matter) and trying to decide if Exchange Online meets all the organization’s requirements.
