Actually it doesn't make sense. There is no such thing as a category not being available on a mailbox. Categories have no association with mailboxes, in the first place. Any category is always "available" because a user can simply type in whatever category text they want to assign to an item. The categories set on an item are stored in the individual item. In other words, if you assign a category of Red to an item, it doesn't matter whether Red is in the master category list. That item will still have a category of Red.
Are you saying that the imported contacts had categories in the old mailbox but those same items have no categories in the new mailbox? I can't explain why that might have happened.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers