Greenfield migration of AD and Exchange

In a nutshell:  We are running Exchange 2010, our internal domain is a namespace that we do not own and therefore cannot by certs for.  I brought up another new domain (that we own) and have built another Exchange 2010 environment that will obviously handle mail for the same domains.  I'm looking to do a department by department migration instead of one fell swoop.   I'm looking at different Exchange migration tools that will help.  I need some sort of syncronization going between the two Exchange servers.  

I have a CISCO Ironport and I figured out how to deliver mail to both Exchange servers if needed.   In testing I found that after I migrate a user they can't email anyone else in the local domain that has not yet migrated because the Exchange server sees that message and realizes it routes mail for that domain and sends an NDR because the account does not exist on the new server yet.  If it did exist on the new server it would deliver however the user would not get it because their outlook ( and computer) is configured to the old.

I you can see my deliema now, has anyone else tried this before?
jonasdurstAsked:
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Simon Butler (Sembee)ConsultantCommented:
The internal name space isn't the issue it should be.
If anyone is running a .local domain they would have the same problem, because you will soon be unable to get certificates that include the internal domain. Yours is the same issue and is resolved easily.
Just reconfigure Exchange to use your external host name internally.
http://semb.ee/hostnames

You are doing a cross forest migration.
Microsoft have documented that process pretty well.
Start here with the Exchange team blog:
http://blogs.technet.com/b/exchange/archive/2010/08/10/3410619.aspx
It will lead you to the resources that you need to plan it.

Simon.
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jonasdurstAuthor Commented:
I'm reading these but am not sere where a tiered or department by department approach is mentioned.   Basically a co-existence during migration.  This requires something like a synching software between the domains
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Simon Butler (Sembee)ConsultantCommented:
You are never going to find documentation that matches your scenario exactly.
You have to adapt what you are seeing in there.
Thus where it mentions the account creation/sync, you need to do that on an OU basis, something like that.

Simon.
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