Multiple Domains - O365 - Exchange Online - Prep for Domain Decom

New Contributor

(Might have posted in the wrong spot so re-posting) :(


Good morning Folks, 


I have read numerous articles on this topic and think I have the right plan in place to accomplish what I need done. I would like to roll it up and present it so this group can poke holes in it before I get myself in a spot I can't get out of. Thanks in advance. 


Domain 1 =  On prem AD - On Prem Exchange 2013 - Hybrid Configuration to O365 (Works like a champ)

Domain 2 = On prem AD - On prem Exchange - Connected with a 2 way trust to Domain 1 (not a sub-domain)


Our stated goals are (in order):

Migrate all user mail accounts in Domain 2 to Office 365, matching what exists in Domain 1 with minimal downtime and manual touch (trying to avoid .PST migrations)

Migrate all users from Domain 2 to Domain 1 with minimal downtime using ADMT

Collapse the old Domain 2 DC's in favor of Domain 1 DC's

Retire the on-prem mail server in Domain 2


How I think we can do this: 

Create Domain 2 as a Trusted Domain in the O365 Tenant

Add Domain 2 as a Trusted Domain on the Domain 1 Hybrid Server

Migrate mailboxes from Domain 2 to O365 with batch migrations

Move user accounts from Domain 2 to Domain 1 using ADMT (to maintain the SID)


These are the reference links used to envision this plan:



Here is where I get a little lost: 

Do I need a separate Hybrid Config in Domain 2 or can the Hybrid config in Domain 1 manage it all? 


When I complete the Hybrid config does it update all public DNS records to route inbound mail to Domain 2 thru O365 - Thru the Domain 1 Hybrid - To the Domain 2 exchange server. (I know I am probably asking this wrong) - OR - Does mail still flow directly inbound to the Domain 2 exchange server until that mail account is migrated to O365?


When we start migrating user mail from Domain 2 to O365, it will be done with their Domain 2 user account in AAD. ( will be added as an object in AAD. - OR - do those users need to be sync'd into AAD before I migrate the mailboxes. (probably asking this wrong as well)


Hope I have provided enough detail. I look forward to feedback. 



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