In Active Directory Users and Computers (ADUC), I copied a disabled user account (User A) to a new employee (User B) who was a replacement. User A had an Exchange 5.5 mailbox that wasn't migrated to Exchange 2003. I was not aware of that until after I set up the account.
Now User B has no associated mailbox. I right-click the user and go to Exchange Tasks, looking for "Create Mailbox,"...
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but all I get is "Remove Exchange Attributes." How do I create a mailbox for User B?
You should be able to simply right-click the mailbox and select "Remove Exchange Attributes." Once this is done, right-clicking should permit you to use "Create Mailbox" to create a new mailbox for User B.