My company (abc.com) is merged with (xyz.com).

Abc.com setup as below:

AD Functional Level Windows 2003

Exchange - Exchange 2003

Xyz.com setup is as below:

AD Functional Level Windows 2003

Exchange - Exchange 2007

Before merger we already planned for AD upgrade to 2008 & Exchange upgrade to 2010

Now management is planning to migrate Exchange environment of abc.com to xyz.com with upgrade to Exchange 2010. This will be achieved by establishing connectivity between abc.com & xyz.com & installing their ADC in our premises & new Exchange 2010 servers will be installed in xyz.com (Which will located in our premises). Users will login using their abc.com credential & then access their mailbox which is hosted in xyz.com.

Need to know if this is feasible & achievable solution.

Also we are planning for AD upgrade of abc.com from 2003 to 2008. Hence post upgrade our domain will be 2008 with Exchange 2003 & xyz.com domain will be AD 2003 & Exchange 2007.

Post this migration we will planning for mailbox migration from abc.com to xyz.com hence is there any compatibility issue while migrating mailboxes from AD 2008 to 2003 i.e. from abc.com to xyz.com.

We will use xyz.com SMTP to relay incoming & outgoing mails.

Need your help to understand it this is feasible solution to implement.

You will need to configure a trust between the ABC and XYZ AD. 

I reccomend you to look at some 3rd party tool to assist you with this. Quest migration manager is one tool that could make your whole life easier.

Management will not go ahead with 3rd party tool as it will involve cost which is additional to Exchange 2010 migration.

Ok then you need to setup a two way trust for the AD doains. 

• Edited by Dagfinn Kolstad 21 hours 4 minutes ago

From an Exchange 2010 perspective, a trust between the two Forests is not a requirement to perform a cross-forest migration.

The trust is requiered to host abc mailboxes in xyz domain while still in their old domain

Ok...but post AD upgrade to 2008 & Exchange upgrade to 2010 in abc.com need to migrate only mailboxes to xyz.com which having AD 2003 & Exchange 2007. How to carryout this migration & is their any backward compatibility issue while migrating only mailboxes from AD 2008 to AD 2003.