Only the Mailbox role can be installed on a cluster server_ClusSvcInstalledRoleBlock
Applies to: Exchange Server
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Microsoft® Exchange Server 2007 setup cannot continue because its attempt to install a role other than the Mailbox Server role on this clustered server failed.
Microsoft Exchange does not support Exchange 2007 roles other than the Mailbox Server role on servers that have the Cluster service installed.
To address this issue, see "High Availability for Exchange 2007" in the Exchange Server 2007 Operations Guide (http://go.microsoft.com/fwlink/?LinkId=68190).