I have company A which just bought company B. Company A has an Exchange 2010 environment, Company B has an Exchange 2007 environment. We want to migrate all mail accounts from Company B to Company A with as little interruption as possible. Company B has multiple locations and exchange servers. I would like to do a slow migration of users in Company B to Company A's exchange environment. My process would be to move the MX record of Company B to Company A. Then set up non-authoritative routing to forward any email to accounts that have not been set up in Company A yet to Company B. My concern is that if I am in Company B, and try to send to an account that used to be in Company B but has been moved to Company A, that it will not send. How do I make sure that emails send back and forth correctly, and that any invalid addresses do not get stuck in a loop between the two environments. The environments are in different domains with a trust between them.
Any help and or documentation on this procedure would be great.