Thanks for the reply Martina
I had 3 mailboxes that were linked, only one of them was originally on a 2003 server and moved to the 2010. It should not have been associated to external content but I can't confirm that.
The other 2 were new mailboxes created on the 2010 server, not sure how they became linked.
No matter your syntax worked and they are now standard user mailboxes.
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