Multi Forest Hybrid Exchange Questions

Regular Visitor

Hi there,


We currently have a Windows Forest (let's call it Forest1) that runs an Exchange environment which is in Exchange Hybrid mode. This uses the email domain The user's UPN is email address removed for privacy reasons
AAD Connect runs on a server in this forest.
All mailboxes have been migrated to Exchange Online.

We have recently acquired a new company which also has a Windows Forest (let's call this Forest2) and an Exchange environment (not in Hybrid, all mailboxes are on premises). The email domain is

We have setup a trust between the forests and plan to sync the accounts from Forest 2 into the existing Azure AD tenant so that we can plan to setup an Exchange Hybrid for the Forest2 mailboxes.
The UPN of the users will be email address removed for privacy reasons

Questions I have:
1. Is this a supported configuration? Can multiple forests and Exchange environments be setup in 1 x Azure/M365 EOL tenant?
2. Since the option to enable Exchange Hybrid Deployment is already selected in Azure AD Connect (in order to setup the Hyrbid for Forest1), will this in any way impact Forest2's Exchange environment when the Forest2 AD accounts are first synced? I'm under the impression that nothing would change in Forest2 until the Exchange Hybrid Config Wizard is run on the Exchange server, is that correct?
3. We need to ensure we don't impact mail flow between Forest1 and Forest2 when we start syncing Forest2 users to Azure. Are we correct in assuming that once the AD accounts have been synced for the first time from Forest2, that we need to create an Exchange connector in EOL to send emails for to Forest2's on premise exchange server until we have the Exchange Hybrid configuration setup?
4. Are there any other items we should consider with the above that could potentially impact either of the environments?

0 Replies