MX records in win2k and Exchange2k

I have a win2k Backoffice server with exchange. We used to have mail for ‘company a’ and ‘company b’ processed through the same server. ‘Company b’ have now moved to their own domain and their MX record has been re-directed through to a third party ‘POP Server’. We now cannot send mail or reply to mail sent through to ‘company b’s’ newdomain domain. They can however, recieve mail from other agencies. Very confusing.