Exchange server 2016 mail flow and transport pipeline

Exchange server 2016 mail flow and transport pipeline

Transport pipeline

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 an Exchange 2016 Mailbox server inside the organization.

Front End Transport service on Mailbox servers   This service acts as a stateless proxy for all inbound and (optionally) outbound external SMTP traffic for the Exchange 2016 organization. The Front End Transport service doesn't inspect message content, doesn't communicate with the Mailbox Transport service, and doesn't queue any messages locally.

Transport service on Mailbox servers   This service is virtually identical to the Hub Transport server role in Exchange Server 2010. The Transport service handles all SMTP mail flow for the organization, performs message categorization, and performs message content inspection. Unlike Exchange 2010, the Transport service never communicates directly with mailbox databases. That task is now handled by the Mailbox Transport service. The Transport service routes messages among the Mailbox Transport service, the Transport service, the Front End Transport service, and (depending on your configuration) the Transport service on Edge Transport servers.

Mailbox Transport service on Mailbox servers   This service consists of two separate services:

 

·         Mailbox Transport Submission service   This service connects to the local mailbox database using an Exchange remote procedure call (RPC) to retrieve messages. The service 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.

·         Mailbox Transport Delivery service   This 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 RPC to deliver the messages.

 

The Mailbox Transport service doesn't communicate with the Front End Transport service, the Mailbox Transport service, or mailbox databases on other Mailbox servers. It also doesn't queue any messages locally.

Transport service on Edge Transport servers   This service is very similar to the Transport service on Mailbox servers. If you have an Edge Transport server installed in the perimeter network, all mail coming from the Internet or going to the Internet flows through the Transport service Edge Transport server. This service is described in more detail later in this topic.

Mail flow from external senders

1.     A message from outside the organization enters the transport pipeline through the default Receive connector named "Default Frontend <Mailbox server name>" in the Front End Transport service.

2.     The message is sent to the Transport service on the local Mailbox server or on a different Mailbox server. The Transport service listens for messages on the default Receive connector named "Default <Mailbox server name>".

3.     The message is sent from the Transport service to the Mailbox Transport Delivery service on the local Mailbox server or on a different Mailbox server.

4.     The Mailbox Transport Delivery service uses RPC to deliver the message to the local mailbox database.

 

Mail flow from internal senders

SMTP messages from inside the organization enter the transport pipeline through 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 Submission service.

·         Through agent submission.

 

Outbound mail flow

 

1.     The Mailbox Transport Submission service uses RPC to retrieve the outbound message from the local mailbox database.

2.     The Mailbox Transport Submission service uses SMTP to send the message to the Transport service on the local Mailbox server or on a different Mailbox server.

3.     In the Transport service, the default Receive connector named "Default <Mailbox server name>" accepts the message.

4.     What happens next depends on the configuration of the Send connector:

 

o    Default   The Transport service uses the Send connector you created to send the message to the Internet.

o    Outbound proxy   The Transport service uses the Send connector you created to send the message to the Front End Transport service on the local Mailbox server or on a remote Mailbox server. In the Front End Transport service, the default Receive connector named "Outbound Proxy Frontend <Mailbox server name>" accepts the message. The Front End Transport services sends the message to the Internet.

 

Internal mail flow

 

1.     The Mailbox Transport Submission service uses RPC to retrieve the outbound message from the local mailbox database.

2.     The Mailbox Transport Submission service uses SMTP to send the message to the Transport service on the local Mailbox server.

3.     Transport service determines the mail destination and route it to the transport service on the mailbox server where the recipient mailbox is residing.

4.     Mailbox transport delivery service receives the message and connect to user’s mailbox database to deliver.