1x Exchange 2010 on-premises hosts 2 domains/companies, they've now split and will be hosted at separate locations.. autocomplete & NDR's

Question

Per the subject. CompanyA and CompanyB shared the same Exchange 2010 Server. They have close ties but have outgrown the shared offices so CompanyB has moved 2 miles down the road. There is no link between the offices.
CompanyA is keeping the original server, CompanyB has moved to new premises with their own Forest/Domain/DC and a shiny new Exchange 2010 Server.
I'll be using new-mailboxexportrequest / new-mailboximportrequest to move all e-mail data to CompanyB and then creating x500 proxy addresses so that at least at CompanyB they can all reply to CompanyB mails. That helps CompanyB continue to be able to respond
to old CompanyB e-mails/meeting requests.
But CompanyA and CompanyB will still want to e-mail each other regularly, I haven't done the move yet but past experience suggests that due to auto-completes (and how Outlook/Exchange functions internally) they're likely to experience a lot of bounce-backs
e-mailing each other, especially responding to pre-split mail.
I'm likely to use NK2Edit and a login script to automatically remove the other company from the autocompletes so new e-mails should go through okay but are there any other solutions for CompanyA responding to old CompanyB mails and meeting requests
(and vica versa?) Special Send Connector with the old X400 addresses? Anything?
Any helps really appreciated.

Answer 1

Hi,
Agree with Jon.Knight. If there is no direct route between company A and company B, they will have to route over the internet.
When you move company B' users to a new forest, the email address will not exist on both sides at the same time then a shared SMTP namespace would work. You can create an SMTP connector on both sides with the local domain defined in the address space.
With this the Exchange server checks locally for the email address and if it does not find it defined locally, it will route the email to the other Exchange server which will in turn deliver it to a local mailbox.  Make sure both sides have this shared
SMTP address space defined in the accepted domains. The address space needs to be listed as an Internal Relay.
Here is a thread for your reference.
Cross Forest Migration mail flow between Exchange 2007 and 2010
http://social.technet.microsoft.com/Forums/en-US/2592bc14-87aa-4bad-a3b2-edbaab547f3c/cross-forest-migration-mail-flow-between-exchange-2007-and-2010
Hope it helps.
If there are any problems, please feel free to let me know.
Best regards,
Amy
Amy Wang
TechNet Community Support