You are the messaging engineer for your company. The company network contains an Active Directory forest that contains three domains. Each domain contains at least one Microsoft Exchange Server 2003 server. All mail-enabled groups are configured as global distribution groups. You plan to transition from Exchange Server 2003 to Exchange Server 2007. You need to recommend a solution that ensures that all mail-enabled groups can be expanded from any Exchange Server 2003 server or any Exchange Server 2007 server. What should you recommend?