Sequence of Exchange 2010 upgrade

Currently we have an ISA 2006 nlb cluster in a DMZ that forwards client requests to an Exchange 2003 OWA nlb cluster which then forwards to an Exchange 2003 active\passive cluster:

Remote client--> ISA 2006 nlb---> OWA nlb--> Exchange backend.

In transitioning to exchange 2010, should I install Exchange 2010 mailbox servers first, or start with installing Exchange 2010 CAS servers ? I don't plan on keeping mailbox and CAS roles on the same machine.
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You need to first get CAS servers installed. Whenever you upgrade, you need to remember that we need to upgrade the CAS server first.

Similarly if there are any new updates to be applied... they need to be applied on the CAS servers itself.
anarineAuthor Commented:
One other thing,when I move mailboxes to the new exchange 2010 mailbox server, will outlook 2003 clients be pointing to the exchange 2010 mailbox server or the cas server ?
Pete LongTechnical ConsultantCommented:
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