Long story short, we are in the middle of a merger. Company A has the SMTP domain for the whole company and Company B has to get new email addresses on companyA.com. Company A has Exchange 2007 and Company B has 2003. My question is, we have a full domain level trust between the 2 organizations. Can I have a single SMTP domain prior to a complete Exchange merger. Can someone at company B have a working companyA.com email address prior to us all being on the same environment. If so, how do you setup cross-domain routing for this kind of traffic. Thank you.

Reading this should give you a good start: http://www.msexchange.org/articles_tutorials/exchange-server-2007/planning-architecture/exchange-2007-smtp-namespace-sharing-different-relay-domain-types.html