When an external user sends e-mail messages to a Microsoft Exchange server that runs the anti-spam features, the anti-spam features cumulatively evaluate characteristics of inbound messages and either filter out messages that are suspected to be spam or assign messages a rating based on the probability that the message is spam.

By default, there is no anti-spam functionality enabled on the Hub Transport server. User must run script \Scripts "install-AntispamAgents.ps1" to enable anti-spam on the Hub, including UI.

For detailed information about anti-spam functionality enablement on a Hub transport server, see How to Enable Anti-Spam Functionality on a Hub Transport Server.

Exchange Server 2007 Service Pack 1 (SP1) adds a new Simple Mail Transfer Protocol (SMTP) event OnResolvedMessage. This event is triggered after all the recipients have been resolved but before the next hop has been determined for each recipient. The OnResolvedMessage routing event enables subsequent events to override the default routing behavior by using the per-recipient SetRoutingOverride method. This extension can be used to implement custom routing rules (such as integration with external anti-spam solution).

The new event OnResolvedMessage will provide custom agents the ability to override the default routing behavior by setting the per-recipient RoutingDomain property. The third party agent will be responsible for determining whether or not the routing behavior should be overridden for each recipient with internal business logic and utilize the per-recipient RoutingDomain property to implement this override. The per-recipient RoutingDomain property value should match the address space to route messages to an SMTP smarthost connector.

Since the per-recipient RoutingDomain property will not be transmitted between Hub transport servers, the Send Connector (with address space matching the RoutingDomain value and used to relay messages to external SPAM server/service) must be locally defined on each Hub transport server where the 3rd party agent is installed.

For more information about transport agents and SMTP events, see Overview of Transport Agents.