Exchange 2007 and higher no longer use SMTP Event Sinks. Exchange 2007 and higher use transport agents for processing messages in the SMTP pipeline. These agents are only present on the Edge Transport server and Hub Transport server. The transport agents installed depend on the role assigned to the Exchange server.
By default, an Edge Transport server installs 10 transport agents, a Hub Transport server installs 2 transport agents and a Hub Transport Server with Exchange 2007 SP 1 installed installs 3 transport agents. When the Microsoft Anti-Spam tools are installed, 6 additional transport agents are installed.
Third-party vendors, like Symantec, can write additional transport agents, which will also access and take action on email being transferred over the server. By default, Symantec™ Mail Security for Microsoft® Exchange Server installs two agents.
It is possible to see the full list of transport agents using the Exchange Management Shell (EMS). Open the EMS and type Get-TransportAgent | fl
Sample output from a Hub Transport server is listed below:
Please note the following from the Symantec™ Mail Security Implementation Guide:
Mail Security automatically installs custom transport agents when you install the product on Hub Transport or Edge Transport servers. The Mail Security transport agents consist of an antispam transport agent and an antivirus transport agent. By default, the Mail Security transport agents are installed with a lower priority than the Exchange transport agents. If you modify your transport agent priorities, ensure that the Mail Security transport agents remain a lower priority than the Exchange transport agents.