Pretty much what alphamikevictor said. I just wanted to add that Microsoft has contradictory information out in this regard, or did. Their Small Business Server (RIP) offering provided an all-in-one domain controller/Exchange/SharePoint server, but aside from that they say:
Microsoft Exchange Server 2013 Setup has detected that the computer
you’re attempting to install Exchange 2013 on is an Active Directory
domain controller. Installing Exchange 2013 on a domain controller
If you install Exchange 2013 on a domain controller, be aware of the
Configuring Exchange 2013 for Active Directory split permissions isn’t supported.
The Exchange Trusted Subsystem universal security group (USG) is added to the Domain Admins group when Exchange is installed on a
domain controller. When this occurs, all Exchange servers in the
domain are granted domain administrator rights in that domain.
Exchange Server and Active Directory are both resource-intensive applications. There are performance implications to be considered when
both are running on the same computer.
You must make sure that the domain controller Exchange 2013 is installed on is a global catalog server.
Exchange services may not start correctly when the domain controller is also a global catalog server.
System shutdown will take considerably longer if Exchange services aren’t stopped before shutting down or restarting the server.
Demoting a domain controller to a member server isn’t supported.
Running Exchange 2013 on a clustered node that is also an Active Directory domain controller isn’t supported.
We recommend that you install Exchange 2013 on a member server.
I can personally vouch for "System shutdown will take considerably longer if Exchange services aren’t stopped before shutting down or restarting the server." In an SBS-style all-in-one environment with no backup domain controller, it can take an hour for the server to shut down while Exchange tries to contact a domain controller (which shut down before Exchange). ("Bueller... Bueller... Bueller... Bueller...")
The Small Business Server all-in-one offering has been replaced with cloud-based services.
Since the Exchange server is already a domain controller and they say, "Demoting a domain controller to a member server isn’t supported," this question is pretty much academic.