Applies to: Exchange Server 2013
Topic Last Modified: 2013-01-08
In Microsoft Exchange Server 2013, mail flow occurs through the transport pipeline. The transport pipeline is a collection of services, connections, components, and queues that work together to route all messages to the categorizer in the Transport service on a Mailbox server inside the organization.
Looking for a list of all mail flow topics? See Mail flow documentation.
The transport pipeline
The transport pipeline consists of the following services:
- Front End Transport service This
service runs on all Client Access servers and acts as a stateless
proxy for all inbound and outbound external SMTP traffic for the
Exchange 2013 organization. The Front End Transport service doesn't
inspect message content, only communicates with the Transport
service on a Mailbox server, and doesn't queue any messages
- Transport service This service runs on
all Mailbox servers and is virtually identical to the Hub Transport
server role in previous versions of Exchange. The Transport service
handles all SMTP mail flow for the organization, performs message
categorization, and performs message content inspection. 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:
the Mailbox Transport Submission service and Mailbox Transport
Delivery service. The Mailbox Transport Delivery service receives
SMTP messages from the Transport service on the local Mailbox
server or on other Mailbox servers, and connects to the local
mailbox database using an Exchange remote procedure call (RPC) to
deliver the message. The Mailbox Transport Submission service
connects to the local mailbox database using RPC to retrieve
messages, and submits the messages over SMTP to the Transport
service on the local Mailbox server, or on other Mailbox servers.
The Mailbox Transport Submission service has access to the same
routing topology information as the Transport service. Like the
Front End Transport service, the Mailbox Transport service also
doesn't queue any messages locally.
Messages from outside the organization enter the transport pipeline through a Receive connector in the Front End Transport service on a Client Access server and are then routed to the Transport service on a Mailbox server.
Messages inside the organization enter the Transport service on a Mailbox server in one of the following ways:
- Through a Receive connector.
- From the Pickup directory or the Replay directory.
- From the Mailbox Transport service.
- Through agent submission.
|If you have an Exchange 2010 or Exchange 2007 Edge Transport server in your perimeter network, Internet mail flow occurs directly between the Transport service on the Mailbox server and the Edge Transport server. For more information, see Use an Edge Transport Server in Exchange 2013.|
The following figure shows the relationships among the components in the Exchange 2013 transport pipeline.
The Transport service on a Mailbox server
Every message that's sent or received in an Exchange 2013 organization must be categorized in the Transport service on a Mailbox server before it can be routed and delivered. After a message has been categorized, it's put in a delivery queue for delivery to the destination mailbox database, the destination database availability group (DAG), Active Directory site, or Active Directory forest, or to the destination domain outside the organization.
The Transport service on a Mailbox server consists of the following components and processes:
- SMTP Receive When messages are received
by the Transport service, message content inspection is performed,
transport rules are applied, and anti-spam and anti-malware
inspection is performed if they are enabled. The SMTP session has a
series of events that work together in a specific order to validate
the contents of a message before it's accepted. After a message has
passed completely through SMTP Receive and isn't rejected by
receive events, or by an anti-spam and anti-malware agent, it's put
in the Submission queue.
- Submission Submission is the process of
putting messages into the Submission queue. The categorizer picks
up one message at a time for categorization. Submission happens in
- Through an SMTP Receive connector.
- Through the Pickup directory or the Replay directory. These
directories exist on the Mailbox server. Correctly formatted
message files that are copied into the Pickup directory or the
Replay directory are put directly into the Submission queue.
- Through a transport agent.
- Through an SMTP Receive connector.
- Categorizer The categorizer picks up
one message at a time from the Submission queue. The categorizer
completes the following steps:
- Recipient resolution, which includes top-level addressing,
expansion, and bifurcation.
- Routing resolution.
- Content conversion.
- Recipient resolution, which includes top-level addressing, expansion, and bifurcation.
- SMTP Send How messages are routed from
the Transport service depends on the location of the message
recipients relative to the Mailbox server where categorization
occurred. The message could be routed to the Mailbox Transport
service on the same Mailbox server, the Mailbox Transport service
on a different Mailbox server that's part of the same DAG, the
Transport service on a Mailbox server in a different DAG, Active
Directory site, or Active Directory forest, or to the Front End
Transport service on a Client Access server for delivery to the
Mail flow documentation
The following table contains links to topics that will help you learn about and manage mail flow in Exchange 2013.
Mail routing describes how messages are transmitted between messaging servers.
Connectors define where and how messages are transmitted to and from Exchange servers.
Accepted domains define the SMTP address spaces that are used in the Exchange organization. Remote domains configure message formatting and encoding settings for messages sent to external domains.
Transport agents act on messages as they travel through the Exchange transport pipeline.
Transport high availability describes how Exchange 2013 keeps redundant copies of messages during transit and after delivery.
Transport logs record what happens to messages as they flow through the transport pipeline.
Moderated transport requires approval for messages sent to specific recipients.
Content conversion controls the Transport Neutral encoding format (TNEF) message conversion options for external recipients, and the MAPI conversion options for internal recipients.
Delivery status notifications (DSNs) are the system messages that are sent to message senders, for example, non-delivery reports (NDRs).
Delivery Reports is a message tracking tool that you can use to search for delivery status on email messages sent to or from users in your organization's address book, with a certain subject. You can track delivery information about messages sent by or received from any specific mailbox in your organization.
This topic describes the size and individual component limits that are imposed on messages.
You use the Queue Viewer in the Exchange Toolbox to view and act upon queues and message in queues.
The pickup and replay directories are used to insert message files into the transport pipeline.
This topic describes the considerations for using an Edge Transport server from previous versions of Exchange in Exchange 2013.