Move mailbox error

[spectra84]

Ex2000 to Ex2007 transition.

All mail servers sit in one domain, all MB's are spread across 3 other domains.

I can move Mb's residing in 2 domains, but not from 1 domain.

Inheritance is turned on.

The error I get:

Error occurred in the step: Opening source mailbox. Failed to open mailbox with error:ClassFactory cannot supply requested class, error code: -1056749262

I have added the domain admin account for the problem domain to the Exch Org group, I have logged into the Ex2007 server as the domain admin, but so far nothing has worked.

Any suggestions?

Thanks!