Company A purchased Company B three years ago, and setup user accts and mailboxes on Exchange 2007. Since Company B is physically located in another city, all clients use Outlook setup using RPC/http. Or they use OWA. Now Company B has their own Exchange 2010 server, and new mailboxes etc. Old email was exported / imported using pst files. The problem is with internal emails, because when Bob@companyB sends an email to Suzy@companyA who is in his adress book, he gets an error because she doesn't exist in his organization. When Suzy emails Bob, Exchange simply dumps it in his old mailbox, even though he won't see it. [unless he uses OWA for the old acct.] External emails work fine. The original plan was to leave the old user emails in place for reference, but since the mail has been imported, it isn't necessary. Disabling a mailbox for a CompanyB user on CompanyA's server didn't help, nor did "hide from exchange address lists. How do I fix this?