AD / Exchange Migration / Directing emails to correct organisation

Two companies are merging together. Both existing companies have one internal AD domain/forest and one Exchange 2010 organisation.

I have created a new AD single domain, single forest (2016) and Exchange (2016) environment for this new company which is authoritative for the new external email domain, and have a two-way trust between all three. I also have a Exchange Federation Trust and Organisation sharing setup - free/busy information is working. Eventually all accounts from the two old domains/forests will be migrated into the new domain, when this happens the old AD domains will be retired.

The new company has a new external email domain name. From day one all users must have an email address on the new domain name. However, we don’t want to migrate the users into the new AD domain at this stage due to workstation builds, access to resources, old apps etc.

I have been led to believe that I need to make the new Exchange environment authoritative for all three email domain names. Then there is something in Exchange to do with send/receive connectors will is intelligent enough to forward incoming emails to the correct old Exchange environments, in the old domains – therefore landing in the users old mailboxes. I have been told I do not need to create any mailboxes or AD accounts in the new domain for this to work.

For example gets forwarded to a mailbox in old domain1 i.e. gets forwarded to a mailbox in old domain 2 i.e.
Can anyone please help?
Matthew WildeInfrastructure ManagerAsked:
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Pete LongTechnical ConsultantCommented:
From an Exchange perspective, I use the cross forest migration technique, which keeps mail flowing between two (or more) Exchange Orgs while the migration proceeds, Ive folded 7 companies into one using this method over a nine month period.

Exchange ‘Cross Forest’ Mail Migrations

I also use ADMT at the same time, for the domain user/computer/group end of the migration

Yes, no need to create user / mailbox in 3rd domain, but you do need contacts in 3rd domain
you need to configure 3rd domain as accepted domain in new environment and MX need to be pointed to this exchange environment

Then at 1st domain and 2nd domain, you need to set 3rd domain as primary SMTP and email flow will work from 3rd domain to other two domains. You also need to make current SMTP domain in 1st and 2nd Ad domain to secondary domains
You need to play on setting up target address in 3rd domain contacts pointing to secondary domain in 1st and 2nd domain so that 3rd domain will receive emails from internet and distribute to 1st and 2nd domain based on target domain through send connector with smart hosts (1st and 2nd domain SMTP servers)
likewise for mail flow between 1st and 2nd domain, you need to configure 3rd SMTP domain as internal relay domain in 1st and 2nd domain

scenario could be something like below
SMTP domain in 1st Ad domain  - (primary) and (secondary)
SMTP domain in 2nd AD domain - (primary) and (secondary)
SMTP domain in 3rd AD domain - (primary)

MX will point to SMTP domain in 3rd AD domain i.e
for every user (contact) in 3rd domain, you need to set corresponding user (1st Or 2nd domain) secondary address as targetaddress

Ex: contact is sitting in domain3, this contact will have as targetaddress, now when user1 receives email from internet in domain3, it will accept email and forward it to SMTP server in domain2
likewise user in domain2 ( when tries to send email to another user (say, in that case domain2 exchange will 1st check if it have this mailbox, if yes mail will be delivered locally, if user2 don't have mailbox, it will forward that mail to domain3 SMTP domain because of internal relay and then 3rd domain exchange server will check if user2 contact exists, if exists it also see that user2 has targetaddress set as and then forward that mail to domain1 SMTP server

If your user quantity is small you can do this manually, however if user base is large, you need some automated script or 3rd party Galsync solution or Microsoft native GALsync solution (MIM) to manage this stuff automatically

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the current mailfow at domain1 and domain2 should remain unaffected with their respective MX records
domain2 would receive emails from internet on his MX and same for domain1 as well

Once your mailfow logic is ready and Gal sync  solution is in place, its time to migrate mailboxes with cross forest technic
you need to use native exchange script (preparemoverequest.ps1) which will convert contacts in domain3 to mail enabled users while keeping targetaddress intact and then you can use ADMT in merge mode to insert SID history from domain1 or domain2
Now enable mailbox for that user in domain3, once you enabled mailbox, it will start receiving emails and it won't forward emails anymore, Infact its not required.
now do remote mailbox move request from domain3 only
note that prepare move request , ADMT and move request all need to be run from domain3 over cross forest trust
Matthew WildeInfrastructure ManagerAuthor Commented:
Thank you so much - this worked perfectly.
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