Exchange 2013/2007 merger

My company has been acquired and I have been tasked with merging the two mail systems.  The end goal is that my company’s domain goes away, however this will take time to complete and the number one priority is to allow users still in the old domain to send and receive mail with the new domain. My company runs Exchange 2013 and the company that acquired us runs Exchange 2007.   They are in the process of upgrading their system to Exchange 2013, but before that will complete, the old company needs the ability to send and receive as the new company.  For all examples, my company will be “olddomain.com” and the company that is acquiring us will be “newdomain.com”.  
My first thought was to just create all olddomain.com users in newdomain.com and give them all 2007 mailboxes.  Since the end game is that olddomain.com goes away, I think we could live with the multiple credentials/sign-ons temporarily… the big downside of that to me is, we’d have to export all mail from the 2013 DB’s to .pst and then import them to 2007.  With over 150 mailboxes and 500GB of mail, this doesn’t seem feasible.  
My next thought was to create all olddomain.com users a mailbox in newdomain.com, then set the mailboxes to forward to the olddomain.com email address.  Then create contacts in each organization for all users in the other.  I like this plan, but the overhead of managing the contacts seems too much.  Having to add a contact in both organizations every time an employee is added… leaves a lot of room for error.  
Lastly, I looked at SMTP Name Space sharing… but I’m not sure if this solution would meet our goals, since I do not want a new namespace, rather to share the existing one.
I’m looking for any suggestions or experiences from anyone with knowledge on this subject.
jcm1223Asked:
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

x
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

AmitIT ArchitectCommented:
This option looks better to me:
My next thought was to create all olddomain.com users a mailbox in newdomain.com, then set the mailboxes to forward to the olddomain.com email address.  Then create contacts in each organization for all users in the other.  I like this plan, but the overhead of managing the contacts seems too much.  Having to add a contact in both organizations every time an employee is added… leaves a lot of room for error.
Simon Butler (Sembee)ConsultantCommented:
Once the source is up to Exchange 2013, a cross forest migration is the way to go. That will save the PST migration bit and you can do the move to the new environment in a more controlled manner.

As that is best done following the guidance with regards to account creation, I would be doing SMTP namespace sharing on both servers as an interim solution.
If you have users who need to start working with the other site closely, then do a manual move of those individuals.

Simon.
jcm1223Author Commented:
@Simon
Sorry, i'm having trouble understanding the documents I am reading regarding SMTP namespace sharing.  It almost looks like I have to create a new namespace to share, rather than share the existing name spaces.  So you are saying that with Name Space sharing I can have users in olddomain.com send and receive mail as newdomain.com, while keeping their mailboxes on the olddomain.com exchange server?  If that is the case, I agree that does sound like the best solution.  I just want to be sure I don't need to make a new domain and make that shared between the two organizations.  Does that make sense what i'm asking?
Simon Butler (Sembee)ConsultantCommented:
All of the instructions will be written from the point of view of doing the work from scratch - a new domain etc.
However if you have an existing domain then you just need to adjust the settings on the Exchange servers to allow that to work.
In order to ensure there isn't a loop, one server needs to retain the domain as Authoritive. The other server is then setup as per the instructions.

Simon.

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Exchange

From novice to tech pro — start learning today.