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Exchange 2007 and 2010 split domain migration

I have company A which just bought company B.  Company A has an Exchange 2010 environment, Company B has an Exchange 2007 environment.  We want to migrate all mail accounts from Company B to Company A with as little interruption as possible.  Company B has multiple locations and exchange servers.  I would like to do a slow migration of users in Company B to Company A's exchange environment.  My process would be to move the MX record of Company B to Company A.  Then set up non-authoritative routing to forward any email to accounts that have not been set up in Company A yet to Company B.  My concern is that if I am in Company B, and try to send to an account that used to be in Company B but has been moved to Company A, that it will not send.  How do I make sure that emails send back and forth correctly, and that any invalid addresses do not get stuck in a loop between the two environments.  The environments are in different domains with a trust between them.  

Any help and or documentation on this procedure would be great.

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Malli BoppeCommented:
This is how I would do.

1.)  Change the mx record from domain B .com and add it as accepted domain in company A and remove it from Company B
2.)for all users in company B add   a new email address called companyB.local and remove the email address for userX@domainB.com. You can do this with email address policy and ADmodify tool.
3.) Create a script  to create mail enabled contacts for company B users in company A .
4.) In company A  create a  Send connector for companyB.local domain to forward email to companyB exchange server.

5.) If you want to move a mail box from company B to company A
a.) You create a PST file of the mailbox  in company B.
b.) Delete the mail enabled contact for the user in the company A.Create  a new user account and mailbox in company A. with correct email addresses
c.) Import the pst file in the new mailbox.
d.)delete the mailbox in company B
norwoodhelpdeskAuthor Commented:
With that process, it would not allow users in Company B to be able to still email to each other via their domainb.com address.  Also, when they emailed people outside of the organization, it would set domainb.local to be their default smtp address.  How would I get around that?
Malli BoppeCommented:
Yeh that's true. Users in company B won't be able to use  domainB address  to send internal emails. But they would be able to send emails using domainB.local.
For Domain B users to send external email you need to use Address Rewriting in the companyA

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