The primary purpose of the Hub role is to route mail within the organization and to apply business policies to those messages. With the Hub role, transport rules are based on Active Directory® objects—users, distribution lists, and so forth. The Hub role also lets you add disclaimers to messages or to journal messages sent to groups of users because you’ll probably want to base those rules on Active Directory membership. For example, you might want to journal all messages sent to DLOfUsersOnLitigationHold or add disclaimers to outbound mail sent by DLOfMembersOfLegalTeam. Although it is possible to run anti-spam agents on the hub server, they are not enabled by default so you’ll need to install them manually by running the install-antispamagents script available in the \scripts directory on the server.
The primary purpose of the Edge role is to route mail into and out of the organization as well as perform message hygiene (anti-spam, anti-virus, content or attachment filtering, and so on) on those messages. The anti-spam functionality is enabled out of the box when you install an Edge server. One of the unique features of the Edge role is that there is no requirement for it to be joined to a Windows® domain—it can be run on a standalone Windows Server® installation that is not part of a Windows domain, but Edge can still be managed as part of the Exchange 2007 organization in your corporate forest.