Exchange Q&A: Mail Flow (1)

1.What is 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 an Exchange Mailbox server inside the organization.

2.What are the services of transport pipeline?

  • Front End Transport service on Mailbox servers;
  • Transport service on Mailbox servers;
  • Mailbox Transport service on Mailbox servers:
  • -Mailbox Transport Submission service;
  • -Mailbox Transport Delivery service;
  • Transport service on Edge Transport servers.

3.How messages from external senders enter the transport pipeline?

Inbound Mail Flow (No Edge Transport servers):

  • A message from outside the organization enters the transport pipeline through the default Receive connector in the Front End Transport service;
  • The message is sent to the Transport service on the local Mailbox server or on a different Mailbox server;
  • 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;
  • The Mailbox Transport Delivery service uses RPC to deliver the message to the local mailbox database.

Inbound Mail Flow (With Edge Transport servers):

  • A message from outside the Exchange organization enters the transport pipeline through the default Receive connector in the Transport service on the Edge Transport server;
  • In the Transport service on the Edge Transport server, the default Send connector sends the message to a Mailbox server in the subscribed Active Directory site;
  • In the Front End Transport service on the Mailbox server, the default Receive connector accepts the message;
  • The message is sent from the Front End Transport service to the Transport service on the local Mailbox server or on a different Mailbox server;
  • 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;
  • The Mailbox Transport Delivery service uses RPC to deliver the message to the local mailbox database.

4.What are the ways SMTP messages from inside the organization enter the transport pipeline through the Transport service on a Mailbox server?

  • Through a Receive connector;
  • From the Pickup directory or the Replay directory;
  • From the Mailbox Transport Submission service;
  • Through agent submission;
  • The message is routed based on the routing destination or delivery group.

5.How messages from internal senders enter the transport pipeline?

Outbound Mail Flow (No Edge Transport servers):

  • The Mailbox Transport Submission service uses RPC to retrieve the outbound message from the local mailbox database;
  • 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;
  • In the Transport service, the default Receive connector accepts the message;
  • The Transport service uses the Send connector we created to send the message to the Internet;

Or,

  • The Transport service uses the Send connector we 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 accepts the message. The Front End Transport services sends the message to the Internet.

Outbound Mail Flow (With Edge Transport servers):

  • The Mailbox Transport Submission service uses RPC to retrieve the outbound message from the local mailbox database;
  • he 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;
  • In the Transport service on a Mailbox server in the subscribed Active Directory site, the default Receive connector accepts the message;
  • The message is sent to the Edge Transport server using the implicit and invisible intra-organization Send connector that automatically sends mail between Exchange servers in the same organization;
  • In the Transport service on the Edge Transport server, the default Receive connector accepts the message;
  • In the Transport service on the Edge Transport server, the default Send connector sends the message to the Internet.

6.What components and processes the Transport service on a Mailbox server consists of?

  • SMTP Receive;
  • Submission;
  • Categorizer;
  • SMTP Send.

7.What components and processes the Transport service on Edge Transport Servers consists of?

  • SMTP Receive;
  • Submission;
  • Categorizer;
  • SMTP Send.

8.What are the types of the routing destinations?

  • A mailbox database;
  • A connector;
  • A distribution group expansion server.

9.What is Delivery groups?

A collection of one or more transport servers is responsible for delivering mail to each routing destination. This collection of transport servers is called a delivery group.

10.What logic Exchange uses to select the routing path for a message?

Calculate the least-cost routing path by adding the cost of the IP site links that must be traversed to reach the destination. If the destination is a connector, the cost assigned to the address space is added to the cost to reach the selected connector. If multiple routing paths are possible, the routing path with the lowest aggregate cost is used.

Note: Size limits on connectors are a factor here. Connectors that are configured with message sizes limits smaller than the size of the message are eliminated from consideration. For more information, see Connector selection in external message routing.

If more than one routing path has the same aggregate cost, the number of hops in each path is evaluated and the routing path with the least number of hops is used.

If more than one routing path is still available, the name assigned to the Active Directory sites before the destination is considered. The routing path where the Active Directory site nearest the destination is lowest in alphanumeric order is used. If the site nearest the destination is the same for all routing paths being evaluated, an earlier site name is considered.

Still have questions? Click here to view Microsoft’s official documentation on Exchange.

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