Don't be a dork - CCR clusters

When you're installing a CCR cluster, you need to first cluster Windows, then do a custom install of Exchange, choosing either to install the Active Clustered Mailbox role (first) or the Passive Clustered Mailbox rold (second). Don't install Exchange assuming that you can go back and create a CCR cluster later.

LCR clusters, OTOH, can be created after Exchange has been installed.

Lesson of the day: Always review your docs before installing. Unless you like watching installs.