Hello and thanks for reading, I am planning a 2003 to 2010 Exchange migration. I need clarification on some points... Proposed migration plan :- install a server with the CAS role and ensure existing mail flow is operating as expected certificate to be created using the creation wizard on the CAS and then applied build 2010 platform behind the CAS array and test fully before migrating any 'live' mailboxes my questions are... will external webmail clients get a cert error when connecting to the array until I get the new cert installed or will the existing 2003 cert be applied? will webmail clients be redirected to the array i.e. the existing address of https://owa.MYDOMAIN.co.uk/exchange/ to https://legacy.MYDOMAIN.co.uk or do I have to amend the DNS? should I just change my OWA record to point to the CAS array? Do I have to have the CAS with the correct cert in place before the mail flow will processed for the 2003 exchange? I want to break the changes into small pieces so that I can implement them and test before moving on. I hope this is clear I have been reading the help files and technet extensively but I am going in circles Cheers, Colm

For co-existence you need to have two unique URLs anyway. Therefore the simplest method would be to just leave Exchange 2003 alone. Introduce the Exchange 2010 CAS server and do its SSL certificate. The existing OWA users will not be impacted if the existing platform (so SSL and DNS) isn't touched. I wouldn't touch your DNS until you have Exchange 2010 mailboxes in place and are ready to change the SSL certificate on Exchange 2003 to the legacy URL (or have the certificate for Exchange 2010 with the legacy URL ready and just need to switch them over). If users with mailboxes on Exchange 2003 hit the CAS server, they are just going to be redirected anyway, so why complicate it. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources