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When you have more than one active directory server on a windows domain they keep copies of the Active directory of the domain on each server. Should one active directory server fail the another on the domain can be told to take over roles.

Question --- Can It be like this for an Exchange Servers? I believe the answer is no but I need to ask to be positive. I believe you can not have more than one Exchange server on a domain and therefore there is no dynamic backup copy like this for exchange. correct?

Ron Whites
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    I think you could do some sort of Exchange 2003 cluster but with newer versions of Exchange they have Database Availability Groups that would do this sort of dynamic fail over. You would need more than one server regardless to do either. Also, with Exchange 2003 you should be able to put different roles on different Exchange servers so you can have more than one Exchange server in a single domain. – Pimp Juice IT Oct 28 '15 at 18:19

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It's been possible to have more than one Exchange mailbox server on a domain for as long as I can remember. As PJ Mahoney notes in their comments, older versions of Exchange have also been designed to work well with Windows Clustering.

Modern versions of Exchange (2010, 2013, 2016) support database availability groups. Rather than traditional Exchange mailbox server clustering, this can instead be thought of as a "simple" way of replicating database changes between "physical" copies of a single "logical" mail database on each DAG member.

As with AD's multiple master model for DCs, this can increase availability and resilience but I'd be very very careful about considering it to be an actual backup.

Rob Moir
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    Per RobM's answer: Quick answer, you can have many Exchange servers on a single domain using DAG. When you expand the schema during Exchange installation into AD, items like mailbox location and server are stored for each user. The CAS server role then does queries to determine where a users mailbox resides and sends it to the appropriate server. The DAG portion allows databases to exist on multiple servers, allowing passive copies to exist in case of a server failure. SQL Server adopted this methodology due to its success with Exchange – Chase Oct 28 '15 at 19:41