How to migrate exchange 2003 mailboxes to a different domain

http://www.microsoft.com/technet/prodtechnol/exchange/Guides/e2k3InterOpMig/bd864901-77e8-4b69-96c8-c076c9b556b3.mspx?mfr=true

This is ultimate and matches your question:

http://www.microsoft.com/downloads/details.aspx?FamilyId=C29487EE-EF0A-49F6-A205-F23E0A98F794&displaylang=en

regards
Gopal Krishna K

Your question is much simpler than the answer....I will give you an overview, then a link to MS.
souds like your migrating to a new domain, not just moving mailboxes.  The below process is of complete transparancy to the users.  If you have a large number of users, follow this method...if you have a small number of users it is easier to use ex-merge.

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You need to setup a domain trust relationship between A and B first.

Password policy on Domain A, must be the same as on B before continuing the next step.

Then you need a domain admin account, that will be migrated from A to B(new) using the ADMT tool (During  all migrations you also must Migrate SID history and Passwords for all users including  the domain admin account your going to use.- ! VERY important !).  This account will be given admin rights in both domains and will be the account you use from this point forward to utilize the Active Directory Migration Tool. Using this tool you will migrate all other user accounts and service accounts into Domain B.  During this process you will reconcile duplicate usernames as well.  Once that is done, you will migrate Computer accounts using ADMT admin tool.  Users cannot be logged onto the machines during this time, and all xp firewalls must be turned off.

Now users are going to be logging into their machines using the Domain B...same username and password....all changes are transparent to the user. Even their desktops look the same as they did before, but they are now under a new domain accessing their mailbox on the old domain server.  This works because they still have the same SID in both domains.

The below process assumes you want to re-use your existing exchange server, but put it on the new domain.  you can skip this step and use the exchange migration wizard if you already have a box on Domain B with exchange 2003.
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Now you will create a "swing" server.  Basically, another win2003 box, with exchange 2003....on Domain A. Using the migration wizard in exchange you will move ALL mailboxes onto the swing server.(temporary).  Now you will uninstall exchange 2003 from the old server, and you will disjoin it from the domain.  Then you will completely wipe this box clean and join it to the new domain B.  Install exchange 2003.  Using the wizard, and the admin account you used
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After a clean move of all mailboxes, you will modify your DNS records so that the old server name resolves to the new server IP address.  Keeps you from having to modify, or re-create everyones profile manually.

Now you probably need to run a logon script on each client....using exprofre.exe....which will correct their exchange settings on their local profiles to point to the new DC+ global catalog server, or else they will show offline and public folders/GAL won't update.

Once this is all done....your clients are now logging into the new domain, and accessing their mailboxes on the new exchange server.

ADMT Tool: http://www.microsoft.com/downloads/details.aspx?familyid=6F86937B-533A-466D-A8E8-AFF85AD3D212&displaylang=en

Using the Exchange Migration Wizard: http://support.microsoft.com/kb/328871

General technet article: http://www.microsoft.com/technet/prodtechnol/exchange/Guides/Ex2k3DepGuide/37de2d3a-d42c-4a8f-af0e-c447db96dc71.mspx?mfr=true