Using general SMTP for multiple domains

We have an Exchange 2010 organization(best.com) and recently we have taken 40 companies (domains). These organizations have exchange 2003 and 2007 environments. We ll migrate all these organizations to Exchange 2010 as soon, but before that, we want to implement and use a general SMTP address(@best.com) for all 40 organizations so that everyone in organization can communicate with the same SMTP.
Does anyone have some experience or a best practical idea? I have thought itself to mail forwarding, but maybe does anyone know another solution.