Moving Exchange 2003 mailboxes to Exchange 2007 or Exchange 2010 in a completely different domain

Our company, let’s call us CompanyA, has been purchased by another company, CompanyB.  These companies are not in the same geographical area and we are not on the same domain or network – 2 completely separate entities still.  

CompanyA is running Exchange 2003 SP2 on Windows Server 2003 Standard.  CompanyB is going to upgrade from Exchange 2007 to Exchange 2010 before the end of 2011.

CompanyB wants to migrate our mailboxes, distribution lists, and mail-enabled public folders to their Exchange server, but are not sure if they want to do this before or after the upgrade.  

Our domain is called and their domain is called  After the migration, we will retain as our primary email address, but also will begin using  

Is there a "simple" way to perform this task?  I’ve researched online and found ideas with Exmerge, which doesn’t sound like it will work between the two different versions of Exchange, and read a little about cross-forest migration, which sounds really “involved”.
I’ve got some other ideas of my own, but wanted to see what the Exchange experts have to say.  

I realize that there are DNS records to take into account, but at this point, I’m more interested in the actual Exchange mailboxes getting moved to the new server and platform.  

Thank you.
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ThorinOConnect With a Mentor Commented:
How many mailboxes are going to be migrated? If not a lot I would manually create them on 2007/2010, export to a PST on the 2003 side using exmerge and import on the 2007/2010 side using the import-mailbox command. You could do this as well with a lot of mailboxes.

The other option like you said would be to do a cross forest trust and migrate. If you are not on the same network you would need to setup a VPN.
In your case migrating to 2007 or 2010 really does not do big difference. Using cross-forest migration you must create trusts between the two AD domains. Using exmerge you simply export the contents of mailboxes to pst files  and then import them to mailboxes created on new server. Exchange 2007 SP1+ uses Import-mailbox, Exchange 2010 Sp1 uses New-MailboxImportRequest cmdlet. On exchange 2010 sp1 you dont need to have installed outlook.

You will have a little more work with migration contents of public folders. Maybe the best option is moving contents to mailbox, then to pst, then to mailbox on new server and back to PF...
Anyway you will need to enable PF on exchange 2007/2010 if you want to use older outlook clients.

At the end it really depends on the number of mailboxes you have to move. With big number of mailboxes you will have a lot of work migrating them thru psts.

Having multiple email domain names is no problem at all.
Deepu ChowdaryCommented:
There are several ways to move the mailbox… like
--> Moving the mailbox through EMC
--> Moving the mailbox through powershell and
--> Moving the mailbox by mode script.

Actually it depends upon the scenario which options needs to be select.
--> If we want to move little number of users mailbox we can go with exchange management console.
--> If we are moving the users mailbox one by one we can go with powershell
--> If we want to move the mailboxes in a bulk then we can go with Powershell scripts.

Whenever we are moving the mailbox lots of thing needs to be considered like BadItemLimit, PreserveMailboxSizeLimit, IgnoreRuleLimitErrors, MaxThread etc.

Go through this link
SolCowAuthor Commented:
Thank you for the feedback.  These suggestions were along the lines I was thinking - export to PST, create mailbox on new server, and import to new mailbox via VPN.  
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