For the record before triggering a mailbox move make sure all your User Email aliases in Active Directory Users have no spaces in them.

Open Active Directory Users and Computers on the SBS2003 Server

Navigate to the user account object and right click and choose properties

Select the "Exchange General" tab and check the Alias field value doesn't contain any spaces.

For example if the Alias field has:

"Company Accounts"

You need to change this to:

"CompanyAccounts"