Sharing SMTP Address Space across forests (Exchange 2003 and 2007)

Hello Experts!

I am working on an inter-forest migration and would like to share my SMTP address space across two AD forests.  The source forest is running Exchange 2003 and the destination forest is running Exchange 2007.

I have some very specific questions for someone who has gone through this process before, as I do not have a test environment for this.  As a guide, I am using this document:

Right now, before making any changes, I have two Recipient Policies set up.  I have the Default Recipient Policy and another custom policy.  The SMTP address space that I want to share is specified in BOTH the Default Recipient Policy and the custom policy.  The custom policy is taking highest precedence.

Also, to make things a little more complicated, my email system is receiving messages sent to it from a post fowarder that is external to my organiztaion.  So, my users really have two email addresses to the public world.  One is the SMTP address space that I maintain and another is the SMTP address space maintained by the company with the post forwarder.

Therefore, and here's the important part, I have gone into each mailbox-enabled user's object individually and added the address maintained by the other company as another SMTP address.  Also, this SMTP address is the one that we want the external world to see, so we've set it as the primary. (for reply purposes, etc)

Okay, now this KB321721 document comes along and says that I need to remove an address from my default recipient policy and then make sure that the new address that I set up is stamped on all my user objects or they can't login to OWA, etc.

The big question is whether this process outlined in this document will (a) overwrite/remove values that are already in the mailbox-enabled user object or (b) set another value as the primary SMTP address.

If another SMTP entry is set as the primary SMTP address, I will be up for a few days straight changing these back manually to reflect the external company's address.

Thanks and please post back if this isn't clear and will do my best to explain in better detail.  I am hoping somebody has been through this document and worked out some of the grey area with experience.

Thanks so much!
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heatfan07Author Commented:
Thanks Shree.  More specific question is that I am wondering what will happen to my mailbox-enabled users when I make the changes to their recipient policies.  I am concerned as to whether it will remove their additional SMTP addresses and/or set a different address as the primary address.

Either or both of these results would require quite a bit of manual time on my part to correct.  I appreciate the response.
Shreedhar EtteCommented:
It will add additional SMTP address

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heatfan07Author Commented:
Good information to know.  Also, what will happen to the primary address that is currently set?

As I mentioned on the original post (which may have been somewhat unclear), my users are maintaining another email address that is managed outside of my Exchange Org.  The mail for this additional email address gets routed to my users through the use of a post-forwarder file, external to my organization.

Therefore, I have manually gone into each mailbox-enabled user and manually placed this address into the E-mail Address tab as an additional SMTP address.  In addition to that, I have set this added address as the primary, so that it will be the default reply address.

I am concerned that going through the process of sharing the SMTP address space *could* change my default reply address.  There are couple of excerpts from the KB 321721 article that cause this concern:

Click to select the This Exchange Organization is responsible for all mail delivery to this address check box, and then click OK.

Note The e-mail address space that you specified must be stamped on all e-mail-enabled objects in Active Directory. In particular, this e-mail address space must be stamped on user accounts that have mailboxes. This enables the users to access the Outlook Web Access virtual server.

--- AND ---

Click OK, and then click Yes when you receive the following message:
The e-mail Addresses of type(s) [SMTP] have been modified. Do you want to update all corresponding recipient e-mail addresses to match these new address(es)?
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