Planned to implementation will span multiple servers, Microsoft bestpractices suggest you install the roles in the following order:
1. Active Directory schema and domain preparation: This can be accomplished from thecommand line or with the graphical installer.
2. Client Access role: I recommend installing the Client Access role because Outlook 2007depends on it to obtain its configuration if you’re using the Auto discover service.
3. Hub Transport role: The rationale for installing the Hub Transport Role before any other isthat mail cannot be delivered without it.
4. Mailbox role: Once your core infrastructure is up and running, I recommend installing theMailbox server so you can test intra-organization functionality and mail delivery. If you’reinstalling more than one Mailbox role in a cluster, this is a perfect time to get the clustercompletely up and running prior to bringing clients into production.
5. Edge Transport role (optional): By this time, your Exchange System should be up andstarting to receive its first batch of spam. What better time to install an Edge Transport role?The advantage to having your Hub Transport up before your Edge Transport is that youcan set the Hub to have a lower-priority MX record while tweaking the configuration of theEdge Transport, thereby ensuring minimal loss of incoming messages.
6. Unified Messaging role (optional): If you plan on leveraging Unified Messaging, It is recommendwaiting until the rest of your core Exchange infrastructure is running in production(preferably for more than a couple of weeks) before installing this role.