Help needed with a messy Office 365 migration and company split

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I need help with a messy Office 365 migration and company split!


We are two separate companies that share a network and server infrastructure and are parting ways in the next few months and I am trying to separate out our organisations into two separate Office 365 tenants.


We have two separate domain names for our emails (e.g. and which are both running on the same exchange server (Exchange 2010).


I have separated out all users in our Active Directory into two Organizational Units (Company1 and Company2).


I have installed Azure Active Directory Sync on the Exchange server and synced the Company1 OU with the Company_1 Office 365 tenant. I have also installed a separate instance of Azure AD Sync on the Domain Controller server and synced the Company2 OU with the Company_2 Office 365 tenant. This all seems to work as each O365 tenant now has all the correct users in.


I have also performed a hybrid migration on the exchange server for Company_1 and have successfully synced my mailbox (I work for company 1) onto O365 and this all seems to be working fine.


Company_2 have now said they are happy to have a cutover migration as their situation for the next year will be such that they may be server-less and so are happy to have all their emails in the cloud without a hybrid migration. Company_1 would like to keep a hybrid migration.


I am now trying to migrate users from Company_2 into their O365 tenant by manually adding the SMTP addresses into their users and then performing a migration to their tenant (as the exchange server is still setup as a hybrid for Company_2).


- Will I be able to do this and then just point their mail DNS to the O365 DNS with no problems?
- And without decommissioning the exchange server?
- Is there a better way to do this?


(Company_1 will still be using the exchange server for the time being and will then be migrating to a new upgraded exchange server in a hybrid environment.)

2 Replies
You could, yet you still need to set up e-mail transport between on-prem and Tenant 2 as long as there's a tenant 2 mailbox on-prem. Another limitation is that Company 2 won't have 'Exchange Hybrid', more of a rich-coexistence setup as AD Connect won't be able to perform things like writeback for Company 2. Define a migration endpoint for Company 2, set up staging of mailboxes for Company 2, then pick a cutover moment for them where you perform things like switch DNS record and finalize mailbox moves after which you can convert users in Tenant 2 to cloud only, and decommission ADConnect 2. That said, the preferred path depends a bit on the population and timeframes, for example if you can't perform a cutover due to numbers or organizing support.


Hi @Michel de Rooij, thanks for the reply. Company 2 are happy to be in Tenant 2 with no AD Sync as they are unsure of their building situation for the next year or so. There are only around 30 users in each company so numbers are not a massive issue it's mainly the logistics of performing a cutover migration for one company while in the middle of, and retaining, a hybrid migration for the other company.


I guess what I'm trying to work out now is: given that I am already half way through a hybrid migration with Company 1 (a hybrid migration which won't be completed now for the next few weeks at least) is it ok to manually do a cutover migration for Company 2 (by manually adding in the necessary info such as the SMTP addresses) while keeping the configuration for the Company 1 hybrid and then at the end simply point the DNS for Company 2 to their O365 tenant and then remove their users from the Exchange server? And would I also need to remove the Azure AD sync for them first or do I do that at the end without it causing issues for the users in the tenant?