Multi Forest Hybrid Exchange - same email address all users

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Current setup is 2 forests with 2 exchange servers, 2 domains linked via trust. Both Exchange servers are completely separate.  

We have been running AAD sync on one member server (in domain a )that syncs domain users from BOTH forests to our Office 365 Tenant. 


Currently the on premise exchange servers each receive mail for their respective domain users, and 

I would like to migrate via minimal exchange hybrid each exchange forest to O365, it seems that I would then need to keep both servers on-premise to manage users etc. 


Once each exchange is hybrid and mailboxes have been migrated for all users - how possible/easy at this point would it be to allow all users to use domain email address ( and to have as alias )

6 Replies

I'm not sure minimal Hybrid is supported with multi-forest scenarios, best contact support to confirm that. Once you get the mailboxes to O365, you should have no problems assigning aliases from either domain though, as they will be in the same organization.


Hi thanks for mentioning that, hopefully I can get this confirmed.


setting an alias would only allow you to receive to that address? I would want all users to have the same email address for sending/receiving. 


Yes, you can ;)

Minimal hybrid supports multi-forest Exchange hybrid deployments. --> have a look at the comments, Tim from Microsoft answered the question.


Alias: you can send email from an address which is the reply address of your mailbox. You can use address rewriting or similar features to accomplish this.

Thanks, that's very promising. So this would mean keeping AAD Connect for syncing local AD account passwords as we currently do with two forests.

Do you know of any good information on how to go about using Aliases and address rewriting to get all users having ?

As you will keep your on-premises Exchange at least for recipient management, you can use it as an address rewrite gateway:


Exchange Online also supports Address Rewrite, which routes outgoing email through an on-premises gateway that modifies the addresses. This feature enables you to hide sub-domains, make email from a multi-domain organization appear as a single domain, or make partner-relayed email appear as if it were sent from inside your organization. Administrators configure custom email routing within the Exchange admin center (EAC).


Hello Dominik.


Unfortunately the address rewriting feature is not supported on Exchange online.

MS expert said recently: "we don’t have any plan to bring Address Rewrite to Exchange Online."


There exists a non-MS cloud service that implements the sender address rewriting function for Office 365/Exchange online. 


Best regards,

Victor Ivanidze