First let’s understand when it happens. After move of mailboxes between 2 exchange stores some times duplicate mailboxes remain .. When you try to delete them you receive error "The operation cannot be performed because this mailbox was already reconnected to an existing user." ID no. c1034ad6. So to delete the annoying mailbox go to active directory users and computers , select the user that holds both mailboxes run “exchange tasks” and select “remove exchange attributes” then go to duplicate Read more