At one time we were one big company that we will call OriginalCompany for simplicity. Later the OriginalCompany split into two companies as part of a sell off resulting in NewCompany and OiginalCompany.
When the split occurred both OriginalCompany and NewCompany kept the same Windows Domain names internally, along with the Active Directory even though the two parts are no longer physically connected in anyway.
Our external internet domain has changed from OriginalCompany.com to NewCompany.com and email is working in all cases but one. When we try to send mail to someone at OriginalCompany.com it never seems to route past the exchange 2003 server. I believe that there is likely a configuration that needs to be changed.