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

[Suhil Nazari]

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

contoso.local AD domain has an exchange server setup with contoso.com to receive emails - the plan is to setup Hybrid Exchange on this domain to migrate mailboxes to 365

fabrikam.com AD domain has Azure AD Connect setup and syncing all users to a 365 tenancy already. 365 is receiving on fabrikam.com

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

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 contoso.com 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?

[#2]

Azure AD Connect can certainly handle a multi-forest sync scenario: https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnect-topologi....

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 (https://technet.microsoft.com/en-us/library/jj873754%28v=exchg.150%29.aspx).

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!