With the current economic climate, we're seeing an uptake in merger and aquisition activity across our customers.  One of the first things organizations want to do in these scenarios is often to rebrand the new aquisition to their own name, including email.  Although its not trivial to accomplish a full email consolidation, there are steps we can take to rapidly update the external email addresses of an organization and provide basic interoperability.  In most cases this can be done even without WAN connectivity.  The example below discusses Exchange to Exchange, but this same concept will work with most email systems. 

 Here are the steps: 

1.       Identify and address any email address collisions

a.       Consideration: how the new-hire process can be modified at each location to ensure no future collisions are created

2.       Identify and address any email policy differences to ensure consistent user experience, including but not limited to:

a.       Message size limits

b.      Anti-spam/content filtering rules

c.       Virus filtering experience (quarantine policies, notification format)

3.       Establish SMTP connectors within each Exchange environment

4.       Import account information from child companies to parent and establish forwarders

a.       Consideration: how you will keep this in sync moving forward if account information changes

5.       Configure child companies as non-authoritative for @parent.com

a.       See http://support.microsoft.com/kb/823158 for details. 

6.       Update child companies to have primary SMTP addresses @parent.com via recipient policy or a script 

a.       Keep any existing SMTP addresses (at least for the short term) for work continuity and applications/subscriptions

7.       Import account information across the organization (as a contact) to each child company to ensure everyone has the full GAL 

This will give you email interoperability.  Once you have WAN connectivity between each site, you can take an additional step -

To enable Free/Busy synchronization – this would allow calendaring between Exchange organizations and show free/busy information.  Use the IORepl tool, available at:

http://microsoft.com/downloads/details.aspx?FamilyId=E7A951D7-1559-4F8F-B400-488B0C52430E

This tool would also be used to consolidate/replicate public folders from the child organizations to the parent environment prior to collapsing those Exchange organizations.