First, yes I understand that Exchange 2003 is no longer supported. :) However, in the real world there are still companies running Exchange 2003 in production. Hopefully not for long.

The client I am at is in the middle of a Exchange 2003 to O365 migration.  They are a few months from wrapping the project up.  I am in the middle of doing an Active Directory upgrade from 2003 functionality to 2012 R2.  Currently all their domain controllers in the forest that Exchange is in are running Windows 2003 and of course it is set at 2003 functionality.  They had a few NT 4.0 domains left that we finally decommissioned during this project.  Yuck!

My question is, I have read that Exchange 2003 isn't supported with 2012 R2 domain controllers.  However, I can not find what will happen if we start to introduce 2012 R2 domain controllers.  The plan is to bring up all of the DC's and then cut over to 2012 R2 functionality. 

Does anyone have or know what exactly will happen if we go ahead with deploying 2012 R2 domain controllers.  I can't find any information documented except for it isn't supported.  We have other projects waiting the AD upgrade to be complete and can't wait for the O365 project to be completed.

Hi Kristopher,

As you mentioned that Exchange 2003 does not support Windows Server 2012 Domain Controllers, which will lead to unexpected issues.

Here is a similar thread below for you:

Adding Windows 2012 DC to 2003 Level Domain (with Exchange Server 2003)

https://social.technet.microsoft.com/Forums/en-US/29301f55-531e-40e7-9a9d-bab4d0b05b33/adding-windows-2012-dc-to-2003-level-domain-with-exchange-server-2003

Best Regards,

Amy