Below is a brief description of how to do a cross forest migration which includes Exhange and all AD accounts. This was done with Server 2008, Exchange 2010 and ADMT 3.2 (Active Directory Migration Toolkit).
Before you begin you would need to download ADMT and PES (Password Export Server):
ADMT 3.2 – http://www.microsoft.com/en-us/download/details.aspx?id=8377
PES Service – http://www.microsoft.com/en-us/download/details.aspx?id=10370
1. Install SQL2008R2 SP1 express on target server (Page 60 on ADMT migration guide).
2. Install ADMT on target domain.
3. Create encryption key from target server from command line: admt key /option:create /sourcedomain: /keyfile: /keypassword:{|*}
4. Install Password server (PES Service) on source server and start service.
5. Create trust between the two domains.
6. Migrate groups using ADMT.
7. Run .\prepare –moverequest to prepare mailboxes.
8. Move mailboxes to new server: New-moverequest
9. Migrate AD accounts using ADMT.
10. Use ADmodify to change that users are not requested to change password at first logon – Can also set via AD per user – Set “PwdLastSet” attribute to –1
11. Migrate user profiles using security translation wizard.
12. Migrate computer accounts.
13. Migrate groups again.
14. Copy GPO’s (Using Cross-Domain copying wizard).
15. Re-add distribution groups on new exchange server.
16. Reconfigure any AD integration devices (Eg: Sonicwall AD integration).