Exchange Migration Between Forests - Mail Routing Issue

BLipman used Ask the Experts™
I am preparing a migration from one domain in forest A to a new domain in forest B.  I have an Exchange 2003 server in A and a new Exchange 2007 server in B.  I have SMTP addresses in forest A with two different namespaces (let's call them and  
Forest A's domain is actually, is another namespace we use because it is shorter and has always been around so I need to properly deliver to both and  This works well in the current configuration.  
When I create a test user in my new forest B (let's call this, I can deliver mail to but not  Let me back up a bit.  I have the old server configured to deliver all internal mail to the new server which cannot be resolved in its namespace.  This lets me send to or, hit the old server, when a user isn't found, it forwards to the new server and delivers.  The way back is where things break down.  For some reason, even though I have and set up as internal relay domains, I can only deliver to (the one that matches my old ad name).  I belive this is because is configured in the new DNS as a stub zone pointing back to my old domain controllers.  The namespace (on the new side) is configured as a primary zone.  I am guessing the new mail server is searching the global catalog and finding my users on the old server and suceeding but I am not sure how to get the other namespace delivering.  
I want to move a few mailboxes a week ahead of time to work out any bugs before I make the change to move all user accounts and workstations into the new domain.  I don't want to sort out mail routing issues when I have all of these other issues to work out.  

I have a kind of strange scenario so feel free to ask more detailed questions and I will answer to the best of my ability.  
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AkhaterSolutions Architect

I don't know if it is only me but I've read what you wrote 3 times and I didn't get it totally.

failing emails are sent from 2003 to 2007 or 2007 to 2003 or what ?

" I have the old server configured to deliver all internal mail to the new server which cannot be resolved in its namespace. " what does this sentence means ?


Sorry, the scenario is kind of complicated and I don't have the best understanding of exchange in the first place.  

I have an old domain and an exchange 2003 organization.  I have a new domain in a new forest with an exchange 2007 organization.  I am trying to move my mailbox from the old to the new.  I exmerge, delete my box from the old, and then do the Import-Mailbox on the new side.  That gets my box in the new domain and exchange org.  

I have all incoming mail from our filtering service coming to the old exchange server.  I have the old server set to "Forward all mail with unresolved recipients to host: 'new mail server'".  The new server has a receive connector (Legacy Forest) to accept this mail.  

The new server has three Accepted Domains:
1) (the name of our new AD) - type Authoritative
2) (the name of our old AD and one of our SMTP namespaces in use) - type Internal Relay
3) (an arbitrary SMTP namespace that we use) - type Internal Relay

So far, when I move my mailbox, I can receive mail from an external system (my gmail account), I can receive messages from a user in the old exchange organization using either or
Where I fail is, signing into the new mailbox as me, I try to send to any of my users on the old server with the namespace (; it works going to  

Is this more clear?  Sorry, I am learning and doing at the same time.  Most articles I come across tell you how to do this within a forest but I have two forests with a trust in between.  My DNS in the new AD has stub records pointing back to my internal DNS in the old domain.  

What else do you need to know/have clarified?
AkhaterSolutions Architect

well it is much more clearer now!

how did you get it to work for ?

in you organization config, create a new Send Connector with address space and cost 1, configure it to use your old exchange server as smart host. it should work
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I have a single send connector in my new environment called Legacy Forest, it alows "SMTP * - cost 1", "SMTP - cost 1", and "SMTP - cost 1".  No flow to have a few MS tickets from our partnership so I called one in.  I am getting in touch with the Exchange 2007 connector team now.  

TIA for the help though, they will probably get me working but if you can figure it out in the mean time all the better.  I am dead in my tracks on a huge project because of this so I didn't have much time to get a DIY/Experts Exchange solution together as I would normally allow.  
AkhaterSolutions Architect

and this smtp connector you have is it using a smart host ?
Yep, it is using a smart host.  So I ended up calling Microsoft and we determined that my old Exchange server had some sort of mail routing issue.  We did verify that my setup was good with the exception of my send connector from Exch07->Exch03.  They told me it was better to have one for *, one for, and one for  
I ended up taking my incoming and outgoing mail services over to my new server.  It was better at routing the traffic and making sure unresolved recipients were being found on the other system.  All is good now.  

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