How to write an Exchange 2013 transport agent

What is a Transport Agent?

Transport agents allow Microsoft, developers in your organization and third-party vendors to hook into the Exchange transport pipeline with their code to process messages (e.g. an antivirus scanner for incoming email messages). Transport agents can process email messages that pass through the transport pipeline in many ways. An agent is a .Net assembly that has to be installed on the Exchange Client Access or Mailbox server. The agent is then loaded by the Exchange Transport service and invoked in the transport pipeline on the specified event. In Microsoft Exchange Server 2013, the transport pipeline is made of three different processes:

Front End Transport service: This service runs on all Client Access servers and acts as a stateless SMTP proxy to route messages to and from the Transport service on a Mailbox server.
Transport service: This service runs on all Mailbox servers and is virtually identical to the Hub Transport server role in previous versions of Exchange. Unlike previous versions of Exchange, the Transport service never communicates directly with the mailbox store. That task is now handled by the Mailbox Transport service. The Transport service routes messages between the Mailbox Transport service, the Transport service, and the Front End Transport service.
Mailbox Transport service: This service runs on all Mailbox servers and consists of two separate services: Mailbox Transport Submission and Mailbox Transport Delivery. Mailbox Transport Delivery receives SMTP messages from the Transport service, and connects to the mailbox database using an Exchange remote procedure call (RPC) to deliver the message. Mailbox Transport Submission connects to the mailbox database using RPC to retrieve messages, and submits the messages over SMTP to the Transport service.

Like the previous version of Exchange, Exchange 2013 transport provides extensibility through the Microsoft Exchange Server 2013 Transport Agents SDK. The Exchange 2013 version of the SDK is based on the Microsoft .NET Framework version 4.0 and allows third parties to implement the following predefined classes: