Find the answer from the release notes:
Required Exchange Software and Licenses—Message Store
Revised April 30, 2010
Caution Do not install any Exchange software, including Exchange System Management Tools, on a Cisco Unity 8.x server on which the Microsoft Messaging API and Collaboration Data Objects (MAPI and CDO) are installed. During Cisco Unity installation, if no Exchange software is installed, the Cisco Unity System Setup Assistant automatically installs MAPI and CDO. If you later install Exchange software, portions of MAPI and CDO are overwritten, and Cisco Unity stops functioning. To determine whether MAPI and CDO are installed on the Cisco Unity server, in the Add or Remove Programs control panel, look for Messaging API and Collaboration Data Objects.
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