Multi Forest in trust - New Hybrid Exchange Setup - Only one Exchange Server

New Contributor

Two seperate AD domains (contoso.local, which are in a forest trust on the same LAN


contoso.local AD domain has an exchange server setup with to receive emails - the plan is to setup Hybrid Exchange on this domain to migrate mailboxes to 365 AD domain has Azure AD Connect setup and syncing all users to a 365 tenancy already. 365 is receiving on


Aim is to migrate Contoso.local mailboxes into the SAME 365 tenancy as


I would like to know if it is possible to add contoso.local into the Azure AD Connect so all users sync up to the 365 tenancy - and then setup Exchange Hybrid on contoso.local to begin mailbox migrations to 365. 


End goal is to get rid of the contoso.local AD domain but keep receiving emails into 365.


I am not sure if the Exchange Hybrid environment will need to stay in that case? And what will happen if the contoso.local domain is removed?


1 Reply

Azure AD Connect can certainly handle a multi-forest sync scenario:

Does Fabrikam have an Exchange hybrid in place already? If so, you'll need to be careful about setting up a second hybrid (for Contoso) to the same Office 365 tenant. Certainly possible though (

Lastly, if your goal is to remove the Contoso AD entirely, you'll need to either (A) migrate the Contoso AD users into the Fabrikam AD forest and then perform a "hard match" with Azure AD Connect, or (B) convert the Fabrikam users to cloud only in Office 365.


To be honest, this is not going to be simple. I would highly recommend reaching out to Microsoft, a partner, or a migration vendor for assistance or guidance. Best of luck!