Q: Can you spread an Exchange DAG (Database Availability Group) between two domains?
Now the story: Say you have a single forest named: Contoso.com. In that forest, you have two child domains: East.Contoso.com and West.Contoso.com. You also have Exchange servers deployed in both East and West domains, but none in the root domain. Is it possible to host some Exchange DAG nodes, in the same DAG, deployed in the East.Contoso.com child domain and others in the West.Contoso.com domain? No.
Exchange does fully support AD site resilience and highly available options, but all within the same domain. Accordingly, it’s best to extend a single DAG across two AD sites, where the nodes are in the same domain. The Exchange Supportability matrix is key to knowing which OS’s are supported and which ones are not, for the different Exchange Server versions.
Yet, there is further discussion about why. It’s not an Exchange issue, but a Windows Operating System (OS) problem. Not until Windows Server 2016 could you even have a cluster across domains: https://blogs.msdn.microsoft.com/clustering/2015/08/17/workgroup-and-multi-domain-clusters-in-windows-server-2016/ Therefore, unless you are on Windows OS 2016 or higher, no cross domain cluster is allowed.
Consequently, your next thought is, well, if someone is running Exchange Server 2016 on Windows Server 2016, then they can just add in an Exchange Server 2019 node to an Exchange 2016 DAG? No. And not yet, and most likely not ever. The Exchange Product Group is aware of the request, but as of this writing, the latest information is, that option is not going to be an upgrade path available. Time will tell, as the future is not yet written.