Delivery Class Throttling

Delivery Class Throttling is an automatic process that was introduced with Exchange 2010 SP1. It is designed to address the following issues that administrators may experience in a messaging environment:

  • Because more resources are required to send messages that have large attachments or that are sent to multiple recipients, other message delivery operations may experience increased latency.
  • A high rate of mailbox delivery operations may reduce the user interactive mailbox experience. For example, users may experience slow refresh or update times when they access their mailboxes.
  • No centralised method is available to control how a specific user may inadvertently affect the resources of a Transport server. Such an effect may occur if the user sends messages that have high delivery costs in terms of number of recipients or total message size or both.

Delivery Class Throttling works by assigning an internal cost to messages based on:

  • Message size
  • Number of recipients
  • Frequency of transmission

As the message is being processed by the Categorizer, messages with a higher cost will be assigned a lower priority and placed in separate MAPI delivery queues.  Messages in a higher prirority MAPI queue will be delivered at a higher rate than those in a lower priority queue (the documented ration is 20:1).

Currently there is no documented way to view or modify this behaviour.