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The Outbound Message Transfer Process

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The Outbound Message Transfer Process

This content is no longer actively maintained. It is provided as is, for anyone who may still be using these technologies, with no warranties or claims of accuracy with regard to the most recent product version or service release.

The following steps describe the transfer of a message from a local Microsoft® Exchange user to recipients on Exchange, on a foreign messaging system, or on both:

  1. The mailbox store receives a message with an attached list of recipients from a local user.
  2. The store driver sends the message to the Advanced Queuing component within the SMTP service on IIS, where it is placed in a pre-categorization queue.
  3. The categorizer resolves both recipient and sender addresses using Microsoft Active Directory®, and expands any mail-enabled groups.
  4. For local addresses, the categorizer sends the message to the information store for delivery to the appropriate mailboxes.
  5. For remote addresses, the message is placed in a post-categorization queue where it is picked up by the routing engine.
  6. The routing engine determines the routes for delivering messages to remote addresses. It forwards the message to remote Exchange addresses using SMTP. If some addresses are on a foreign messaging system, the routing engine assigns the message to a gateway that handles the address type of the recipient and passes the message to the message transfer agent (MTA).
  7. The MTA wraps the message in a message transfer envelope and places it in the MTS-OUT folder for delivery through the gateway.
  8. The gateway must take messages from MTS-OUT, translate them into the foreign system format, and deliver them to their intended recipients in the foreign system.

Note  The MTA continues to deliver messages to a gateway even if the gateway service is paused. This can cause messages to collect in the delivery queues and to overflow.

The message transfer envelope (MTE) of a message contains information about the recipients to whom the gateway must deliver the message. Exchange Server puts all addresses that must be handled by the gateway into the recipient list of an MTE. The properties that can be set on an MTE are described in Using Message Transfer Envelope Properties.

There are three recipient lists associated with each message that a gateway handles: the original list that came with the message, the list sent to the MTA, and the list on the MTE. The list on the message itself contains all the recipients to whom the message is addressed. Some of these recipients might have addresses on the local server, within the routing group, or on messaging systems not associated with the gateway. The recipient list that is sent to the MTA is always a subset (although sometimes a complete subset) of the original list.

In Exchange Server 5.5, the MTA routed all non-local addresses on the original message list. In Exchange 2000 Server, the original list is processed by the SMTP transport engine. The original message list is no longer passed to the MTA. The MTE list that the MTA formulates contains only those recipients to whom the gateway must deliver the message. These recipients are called Responsibility-TRUE. For more information, see The Responsibility Attribute.

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Build: June 2007 (2007.618.1)

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