It seems a lot of organizations combine an Exchange Server upgrade with an Active Directory upgrade in a single project. Aside from my own customers in the past who have planned to do both at the same time, I’m often asked by readers here which of the two should be upgraded first.
Whether you upgrade Exchange or Active Directory first is really up to you. There might be environmental factors that you need to take into account, or other compatibility issues other than Exchange to configure, but most of the time I find there’s no advantage either way.
Usually the possible Exchange upgrade paths (e.g. 2007 -> 2013, or 2010 -> 2016) overlap in terms of minimum and maximum Active Directory requirements/compatibility. The Exchange Supportability Matrix explains the compatibility for currently supported versions of Exchange. If you need to know compatibility for anything that is no longer supported, then you might need to dig a little deeper on TechNet (e.g. Exchange 2003).
That said, one thing I do always recommend is to perform the two upgrades as separate pieces of work. even if they are part of the same overall project. It’s no fun trying to troubleshoot issues in an environment where multiple changes are occurring at the same time.