Decommission Existing Exchange Servers
After you have verified a successful migration, you should wait several weeks before you decommission any legacy Exchange Server in your environment. As described in the section Allow all Outlook clients to connect to the new Exchange Server that follows, this transition period is required to:
Ensure that all the Microsoft Outlook® clients on your network have connected to the new Exchange Server at least once.
Ensure that mail flow in your Exchange Server organization does not depend on services that are running on a legacy Exchange Server.
If you are migrating from a Windows SBS environment, you must decommission your existing Exchange Server in Windows SBS within seven days of installing Windows EBS (or 21 days, if you have updated Windows SBS to allow the longer migration time). For more information about migrating from Windows SBS to Windows EBS, see the Microsoft Web site (https://go.microsoft.com/fwlink/?LinkId=123374).
If you are in doubt or need to troubleshoot an issue in your messaging environment, do not decommission any of your existing Exchange Servers.
Decommissioning your legacy Exchange Servers requires careful preparation and planning. You should consider the following:
Back up a legacy Exchange Server before you decommission it.
Decommission your back-end Exchange Servers before your front-end Exchange Servers. If you have an Exchange Server that hosts no mailboxes, do not decommission it until you have decommissioned all of your other Exchange Servers.
Decommissioning your last legacy Exchange Server requires several additional steps. See the Exchange Server TechCenter article at the Microsoft Web site (https://go.microsoft.com/fwlink/?LinkId=99646) for details.
Allow all Outlook clients to connect to the new Exchange Server
When a Microsoft Outlook client first attempts to connect to an Exchange Server after you move the mailbox that it is looking for, the following happens:
The Outlook client contacts your original Exchange Server, expecting to find its mailbox there.
Your original Exchange Server tells the Outlook client that its mailbox has moved to the Messaging Server.
The Outlook client contacts the Messaging Server and finds its mailbox.
In the future, the Outlook client will contact the Messaging Server first (it will not contact your original Exchange Server).
Because Outlook clients will not be able to find their mailboxes the first time they connect to an Exchange Server after the mailboxes are moved (unless your existing Exchange Server is still operational), do not decommission your existing Exchange Servers until all the Outlook clients on your network have connected to the Exchange Server once. Each user who uses Outlook must access their mailbox at least once from each computer they use. There is no easy way to determine when this has happened, so use this methodology:
Decide a reasonable number of weeks (N weeks) in which you think all users who use Outlook will connect to the Exchange Server once. Remember to factor in computers that are shut down because employees are on vacation, computers that are not operational or are being serviced, computers in branch offices, and computers that are used by employees who are traveling (such as field agents or sales people).
Wait N weeks, leaving your existing Exchange Servers fully operational during this time.
Disable your existing Exchange Servers by stopping all Exchange Server services, but leave each server on and operational: On your existing Exchange Servers, in Administrative Tools, open the Services console, and find all services that have a name beginning with “Microsoft Exchange” or “Exchange.” Use standard procedures to stop and disable the services.
If users experience e-mail issues, start the services on your existing Exchange Server to check the issue. If it is fixed, then disable the services again and see if the issue remains fixed. If the issue does remain fixed, leave the existing services disabled.
Troubleshoot further by using the links in the Additional References later in this document or the tools that are available in the Exchange Management Console (for example, the Mail Flow Troubleshooter).
After three weeks with no reported e-mail issues that had to be fixed by starting the services on an existing Exchange Server, you are ready to decommission your existing Exchange Servers. It is recommended that you keep a backup of your original Exchange Servers.