How Messages Are Routed In An Exchange Organization

In a Microsoft Exchange Server messaging environment, you must deploy a Hub Transport server role in each Active Directory site where a Mailbox server role is installed. Hub Transport servers deliver all messages in a Microsoft Exchange Server 2010 organization, including messages sent between two recipients with mailboxes located in the same mailbox database in the same site as well as between Active Directory sites.

The following process describes how a Hub Transport server delivers mail within a single Active Directory site.

  • The message flow begins when a message is submitted to the message store on an Exchange Server 2010 Mailbox server role. If the client is a Microsoft Office Outlook 2007 client, the message is submitted using Messaging Application Programming Interface (MAPI), and the message is written directly to the Outbox in the user’s mailbox.
  • When the Microsoft Exchange Mail Submission service detects that a message is available and waiting in an outbox, it picks an available Hub Transport server in the same Active Directory site and submits a new message notification to the store driver on the Hub Transport server.
  • The store driver on the Hub Transport server retrieves the message from the Mailbox server.
  • The store driver uses MAPI to connect to the user’s outbox and collect any messages that need to be delivered.
  • The store driver submits the messages to the sub-categorizer queue and moves a copy of the message from the user’s outbox to his Sent Items folder.
  • While the message is passing through the Hub Transport server role, the server can use transport agents to modify the message or the message flow. For example, transport agents can apply custom routing or journaling rules, or perform antivirus filtering.
  • For messages destined to arrive at a Mailbox server in the same Active Directory site, the store driver places the message in a local delivery queue and delivers the message through MAPI to the Mailbox server role. The Outlook 2007 client retrieves the message from the Mailbox server.

You read how messages are routed within an Active Directory site. You will now read how messages are routed between Active Directory sites.

  • When a message is addressed to a recipient in the same Exchange Server organization but in a different Active Directory site, the following process takes place.
  • In this scenario, a user with a mailbox on MBX1 is sending a message to users with mailboxes in other Active Directory sites. If the user with a mailbox on MBX1 sends a message to a user with a mailbox on MBX3, MBX1 will forward the message to a Hub Transport server in the same site.
  • The Hub Transport server performs recipient resolution and queries Active Directory to match the recipient e-mail address to a recipient account. The recipient account information includes the fully qualified domain name (FQDN) of the user’s Mailbox server.
  • The server determines the Active Directory site of the user’s Mailbox server. In a default configuration, the local Hub Transport server opens a Simple Mail Transfer Protocol (SMTP) connection to the remote Hub Transport server in the destination site and then delivers the message.
  • After a Hub Transport server in the destination Active Directory site receives the message, it forwards the message to the appropriate Mailbox server in the destination Active Directory site.
  • If the message has multiple recipients whose mailboxes are in different Active Directory sites, Exchange Server 2010 uses delayed fan-out to optimize message delivery. If the recipients share a portion of the path, or the entire path, then Exchange Server sends a single copy of the message with these recipients until the bifurcation point. Thereafter, the Exchange Server bifurcates and sends a separate copy to each recipient.
  • If a Hub Transport server cannot deliver a message to a Hub Transport server in the destination site, the source Hub Transport server attempts to deliver the message to a Hub Transport server in the last site before the destination site, along the least-cost routing path. The Hub Transport server queues the messages in that Active Directory site, and the queue is set in a retry state. This behavior is called queue at point of failure.
  • When the Hub Transport server in the destination site is available, the message is delivered from the Hub Transport server where the message was queued.
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