I believe I understand what is going on but need some help anyway.
Here is my situation, I originally had one company that we will call company A. We were running Exchange 2007 SP1 on Win2K3 boxes. I had the CAS, hub transport , and store on three separate boxes. Company A split into two companies so we have now have Company A and Company B. Company B people still use Company A's AD infrastructure and log on to the network using their Company A domain accounts.We added a secondary SMTP address for the new Company B domain on their Company A mailbox.
Company B now creates accounts on the Microsoft BPOS hosted Exchange Server, which I will refer to . We created a contact that pointed to the new BPOS Exchange mailbox for each user. They created an external MX record that points to the new BPOS mailboxes so emails sent outside of Company A's domain go straight to the cloud. Internal emails from Company A to Company B were routed through our Exchange Servers up to the BPOS cloud.
Company A builds a new Exchange Server 2007 SP2 box and migrates just the Company A users and mail objects to the new server. Following proper procedure and best practices, we completely uninstall the old Exchange server roles and delete all of the Company B mailboxes, contacts, etc... so for all intents and purposes Company B is just another email domain outside of our organization.
When I open Outlook and try to send to a user in Company B using the original Company B address I get an NDR. I know that this is because it is a cached entry in my N2K file; however, i thought that since I removed all references to the Company B email domains in the Company A AD structure, it would go out to the Internet and resolve using the external Company B MX record. The NDR looks like it is looking for a LegacyDN entry on the mailbox because it says it is trying to resolve the original Company A email address. I would attempt to put an x500 address on the Company B users, but they no longer have mailboxes on my domain.
So after that rambling screed, where in the heck is Exchange still trying to resolve to the Company B users original Company A email account? What did I miss in either Exchange or AD? And finally, why can't emails sent from Company A to Company B just resolve using the the Company B Internet MX record like everyone else instead of trying to resolve internally and throwing an NDR.
Thanks in advance for your help.