Export (0) Print
Expand All
4 out of 6 rated this helpful - Rate this topic

Transport Agents

 

Applies to: Exchange Server 2013

Topic Last Modified: 2012-09-25

Transport agents let you install custom software that is created by Microsoft, by third-party vendors, or by your organization, on an Exchange server. This software can then process email messages that pass through the transport pipeline. 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 mailbox databases. 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:

  • SmtpReceiveAgent
  • RoutingAgent
  • DeliveryAgent

When complied against libraries in the SDK, the resulting assemblies are registered with Exchange 2013, which loads the agents and invokes their event handlers during specific stages of the SMTP sessions or message processing. These stages, or events, are part of the agent definitions. The agent registration information is stored in an XML configuration file.

The following list explains the requirements for using transport agents in Exchange 2013.

  • The Transport service fully supports all the predefined classes in the SDK, and therefore any third party transport agents written for the Hub Transport server role in Microsoft Exchange Server 2010 should work in the Transport service in Exchange 2013.
  • The Front End Transport service only supports the SmtpReceiveAgent class in the SDK, and third party agents can't operate on the OnEndOfData SMTP event.
  • The Mailbox Transport service doesn't support the SDK at all, so you can't use any third party agents in the Mailbox Transport service.

Support for legacy transport agents based on versions of the .NET Framework prior to version 4.0 isn't enabled by default, but you can enable it. For instructions, see Enable Support for Legacy Transport Agents.

Contents

Updates to transport agent management

Transport agents and SMTP events

Built-in transport agents

Due to the updates to the Exchange 2013 transport pipeline, the transport agent cmdlets need to distinguish between the Transport service and the Front End Transport service, especially if a Client Access server and a Mailbox server are installed on the same physical server. For more information, see Manage Transport Agents.

Transport Agent management cmdlets manipulate a configuration file located at %ExchangeInstallPath%TransportRoles\Agents. For the Transport service, the file is agents.config. For the Front End Transport service, the file is fetagents.config. Both files use the same format as in Exchange 2010.

Return to top

Note that the information in this section is identical to the Exchange 2010 topic Understanding Transport Agents. In Exchange 2013, the categorizer exists only in the Transport service. SMTP Receive exists in the Front End Transport service, the Transport service, and the Mailbox Transport Delivery service.

Transport agents use SMTP events. These events are triggered as messages move through the transport pipeline. SMTP events give transport agents access to messages at specific points during the SMTP conversation and during routing of messages through the organization. The following tables list the SMTP events that provide access to messages in the transport pipeline.

SMTP Receive events

Sequence SMTP event Description

1

OnConnect

This event is triggered upon initial connection from a remote SMTP host.

2

OnEhloCommand

This event is triggered when the EHLO SMTP verb is issued by the remote SMTP host.

3

OnHeloCommand

This event is triggered when the HELO SMTP verb is issued by the remote SMTP host.

4

OnAuthCommand

This event is triggered when the AUTH SMTP verb is issued by the remote SMTP host.

5

OnEndOfAuthentication

This event is triggered when the remote SMTP host has completed authentication.

6

OnMailCommand

This event is triggered when the MAIL FROM SMTP verb is issued by the remote SMTP host.

7

OnRcptToCommand

This event is triggered when the RCPT TO SMTP verb is issued by the remote SMTP host.

8

OnDataCommand

This event is triggered when the DATA SMTP verb is issued by the remote SMTP host.

9

OnEndOfHeaders

This event is triggered when the remote SMTP host as completed submitting the email message headers.

10

OnEndOfData

This event is triggered when the remote SMTP host issues <CRLF>.<CRLF>, which indicates the end of data.

**

OnHelpCommand

This event is triggered when the HELP SMTP verb is issued by the remote SMTP host. This event can occur at any time after the OnConnect SMTP event and before the OnDisconnect SMTP event.

**

OnNoopCommand

This event is triggered when the NOOP SMTP verb is issued by the remote SMTP host. This event can occur at any time after the OnConnect SMTP event and before the OnDisconnect SMTP event.

**

OnReject

This event is triggered when the receiving SMTP host issues a temporary or permanent delivery status notification (DSN) code to the sending SMTP host. This event can occur at any time after the OnConnect SMTP event and before the OnDisconnect SMTP event.

**

OnRsetCommand

This event is triggered when the RSET SMTP verb is issued by the sending SMTP host. This event can occur at any time after the OnConnect SMTP event and before the OnDisconnect SMTP event.

11

OnDisconnect

This event is triggered upon disconnection of the SMTP conversation by either a receiving or sending SMTP host.

Categorizer events

Sequence Categorizer event Description

1

OnSubmittedMessage

This event is triggered upon submission of a message into the Submission queues on the receiving SMTP host.

2

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.

3

OnRoutedMessage

This event is triggered after messages have been categorized, distribution lists have been expanded, and recipients have been resolved.

4

OnCategorizedMessage

This event is triggered when the Categorizer completes processing the message.

Return to top

Most of the built-in transport agents included in Exchange 2013 are invisible and unmanageable by the transport agent cmdlets. All built-in transport agents in the Front End Transport service and the Mailbox Transport service are invisible and unmanageable. A few of the built-in transport agents in the Transport service are visible using the Get-TransportAgent cmdlet.

Return to top

 
Did you find this helpful?
(1500 characters remaining)
Thank you for your feedback
Show:
© 2014 Microsoft. All rights reserved.