I have been asked this question and my answer have been "It depends". SO I decided to write what I think on this forum and read some experts opinions. - Environment: Windows 2008 r2 domain with one Exchange 2007 server. CAS role faces the Internet - Scenario: Coexistence 2007 & 2010 servers Planning: -Backup of System state on the domain controller with the schema admin role -Backup of the current Exchange 2007 databases Broad steps: 1- Add two Exchange 2010 servers(CAS/HUB , Mailbox) 2- Add legacy.contoso.com record to external and internal DNS pointing to new CAS/HUB 2010 server 3- Change firewall to point current Public IP for mx record(mail.contoso.com) and other services(OWA, IMAP) to CAS 2010 server 4- Change firewall to point new Public IP for legacy.contoso.com to CAS/HUB 2007 server 5- Modify current RECEIVE connector on CAS/HUB 2010 server with the appropiate permissions 6- Modify current SEND connector to send only thru the CAS/HUB 2010 server 7- Disable Outlook Anywhere in CAS/HUB 2007 server 8- Update External URLs on CAS/HUB 2007 server with the legacy.contoso.com host name for the virtual directories: OWA, EWS and ActiveSync 9- Update External URLs on CAS/HUB 2010 server with mail.contoso.con host name for virtual directories: OWA, EWS and ActiveSync. 10-Enable Outlook Anywhere in CAS/HUB 2010 server NOW here it comes the UNDO under these scenarios: Email not flowing, Outook Anywhere not working, Redirection failing Obviously I will try to troubleshoot first, if no luck then I will proceed to UNDO to some extent, GOAL: Mail flow and services back to 2007 server 1- Change firewall to point current Public IP for mx record(mail.contoso.com) and other services(OWA, IMAP) BACK to CAS 2007 server 2- Modify SEND connector to send thru CAS/HUB 2007 server 3- Modify internal DNS records to point back to CAS/HUB 2007 server 4- Enable Outlook Anywhere back in 2007 server and disable it in 2010 server 5- Update External URLs on CAS/HUB 2007 server BACK to mail.contoso.com host name for the virtual directories: OWA, EWS and ActiveSync I believe the above 5 steps should do it for that GOAL. Now if they want me to remove the Exchange 2010 servers then additional steps obviously will need to be performed involving uninstalling, and even System state restore depending on the demands. QUESTION: WHAT do you think of my assumptions. Are they correct? please feel free to ADD, Substract steps and share your point of view. Even better if you add more on the scenario of going back to how it was BEFORE adding the FIRST Exchange 2010 server.

2. Legacy external dns record should point to your old server (CAS) 3. You don't need to change the mailflow yet if you don't want to, I would change that when the migration/transition is done, don't change the outgoing mailflow either This is just my personal point of view Feel free to comment etc Jonas Andersson MCTS: Microsoft Exchange Server 2007/2010 | MCITP: EMA 2007/2010 | MCSE/MCSA Blog: http://www.testlabs.se/blog

Jonas, On Step 2 I meant to write "CAS/HUB 2007" server. Thanks for pointing that out. What do you mean your step "3" ? Is this on the planning or the UNDO section? When I setup coexistence I make CAS/HUB 2010 the new point of connection for sending, receiving and redirecting to Exchange 2007 server. This would get me closer to the final picture down the road which is Exchange 2010 servers by itself. PLease clarify if you have time.

As I know, there’s no official article for “undo” procedure I would suggest you to follow the upgrade checklist for exchange 2007 below, reversely. So, it seems that you haven’t mentioned to move OAB generation back Checklist: Upgrading from Exchange 2007 Notes: After you moved all services back to exchange 2007, please turn off the exchange 2010 servers, verify if there’s any malfunction in the organization Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.

Any update?Please remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.