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Migrating domain and Exchange environment for two companies - need to remove old company from Exchange environment

We migrating an entire domain and Exchange environment to a new datacenter.  Two companies exist in this environment.  Only one plans to stay at the new datacenter.  Due to this we need to remove all instances of the old company in the Exchange 2013 environment.  In general what steps do I need to take?  Half of the users are with the old company and may be purged.  I just need to make sure all Exchange references to the old company domain and respective routing are gone.
ExchangeWindows Server 2012Active Directory

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8/22/2022 - Mon

In that case if one company remains in old data center, for new data center you need to deploy everything new?in cluding exchange AD and so forth?

what about exchange, you will setup your new AD and exchange at new data center or you will replicate existing network to new datacentre by deploying additional DCs and exchange servers for existing domain?

We are moving all old servers.. This included the domain controller and the Exchange server.  But.. The Exchange server has both companies.
 We need to remove the old company and leave the other company for if they try to email the old company it will try to send it within the Exchange environment.  They'll need to delivery mail externally to email the old company....  We need guidance on how to remove the old company email domain from the Exchange environment...

to separate, at any one side has to build new AD domain along with new email system (may be exchange / O365 exchange online)

you need to deploy all together separate AD and exchange for old company / new company and do cross forest users and mailbox migration

U can achieve entire migration with native tools such as ADMT and exchange powershell scripts for mailbox migration or hybrid migration approach in case of O365

this is 5000 feet view, needs careful architecting, planning and you has to undergo migration - co-existence phase
I started with Experts Exchange in 2004 and it's been a mainstay of my professional computing life since. It helped me launch a career as a programmer / Oracle data analyst
William Peck

I don't think I explained it properly.  Each company has their own email domain.  They both exist inside of the same Exchange environment.  We are using vmware snapshots to snapshot the old Exchange and domain environment to the new data center.  Two companies exist within this environment.  Only one of those two companies will be using the environment that will snapshotted to the new data center.  We need instructions on how to remove the old company's email domain from the Exchange environment.

OK, thanks for explaining scenario in exact words
So you will use same AD and exchange infra but in isolated environment, correct?

in that case once yo u moved to another datacentre, you need to point your domain MX to your new datacentre exchange public IP, change your exchange dns records to point to exchange new public IP and once mail flow is started for your domain to / from internet, you need to simply delete old company users, this action will delete their mailboxes as well
Finally remove old company email domain from accepted domain on exchange server
This will ensure that whenever you send emails to old company, it will travel through internet

likewise old company should delete new company users along with mailbox and delete new company domain from accepted domain

Only thing I am not sure how good VMware snapshot technology is to clone existing VMs specially exchange and AD VMs and mount in separate datacentre

The two environments will split up and will never talk again.  We will be using public MX records to talk to them and they'll use public MX records to talk to us.  We just need to clean up Exchange on the new data center side for we are not responsible for the old data center side.
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When you delete an AD user and their Exchange mailbox typically it's in a disconnected state.  Will Exchange let me remove the domain from the accepted domain setting if the recipient policy has that domain linked to a disconnected mailbox?  I assume we'll need to make sure that users don't have the domain to be removed as an associated domain/email alias to their mailbox, right?

Yes that's right