Exchange is the software that provides the back end to an integrated system for e-mail, calendaring, messaging, and tasks. Outlook is the application installed on your computer that can be used to access the Exchange system.
Exchange only supports up to 20,000 messages in a single folder. If you have more than that in one folder, the system will not perform as well for you. We recommend you archive messages from that folder, or create another folder and move some of the messages to the new folder.
