Types of Queues in Exchange Server

The routing of a message determines the type of queue in which a message is stored. The following types of queues are used in Exchange 2010:

• Submission queue   A persistent queue that's used by the categorizer to gather all messages that have to be resolved, routed, and processed by transport agents. The categorizer is a component of Exchange transport that processes all inbound messages and determines what to do with the messages based on information about the intended recipients.

All messages that are received by a transport server enter processing in the Submission queue. Messages are submitted through a Receive connector, the Pickup directory, or the store driver. The categorizer retrieves messages from this queue and, among other things, determines the location of the recipient and the route to that location. After categorization, the message is moved to a delivery queue or to the unreachable queue. Each Exchange 2010 transport server has only one Submission queue. Messages that are in the Submission queue can't be in other queues at the same time.

• Mailbox delivery queue   The mailbox delivery queues hold messages that are being delivered to a Mailbox server by using encrypted Exchange RPC. Mailbox delivery queues exist on Hub Transport servers only. Mailbox delivery queues hold messages that are being delivered to mailbox recipients whose mailbox data is stored on a Mailbox server that's located in the same site as the Hub Transport server. More than one mailbox delivery queue can exist on a Hub Transport server. The next hop for a mailbox delivery queue is the distinguished name of the mailbox store.

• Remote delivery queue   Remote delivery queues hold messages that are being delivered to a remote server by using SMTP. Remote delivery queues can exist on both Hub Transport servers and Edge Transport servers, and more than one remote delivery queue can exist on each server. Each remote delivery queue contains messages that are being routed to recipients that have the same delivery destination. Remote delivery queues are dynamically created when they're required and are automatically deleted from the server when they no longer hold messages and the configurable expiration time has passed. By default, a remote delivery queue is deleted three minutes after the last message has left the queue. The next hop for a remote delivery queue is an SMTP domain name, a smart host name or IP address, or an Active Directory site name.

• Poison message queue   The poison message queue is a special queue that's used to isolate messages that are detected to be potentially harmful to the Exchange 2010 system after a server failure. Messages that contain errors that are potentially fatal to the Exchange system are delivered to the poison message queue. This queue is typically empty, and if no poison messages exist, the queue doesn't appear in the queue viewing interfaces. The poison message queue is always in a ready state. By default, all messages in this queue are suspended. The messages can be deleted if they're considered to be harmful to the system. If the event that caused the message to enter the poison message queue is determined to be unrelated to the message, delivery of the message can be resumed. When delivery is resumed, the message enters the Submission queue.

• Unreachable queue   Each transport server can have only one Unreachable queue. The Unreachable queue contains messages that can't be routed to their destinations. Typically, an unreachable destination is caused by configuration changes that have modified the routing path for delivery. Regardless of destination, all messages that have unreachable recipients reside in this queue.

Queue database files

All the different queues are stored in a single ESE database. By default, this queue database is located on the transport server at

 %ExchangeInstallPath%TransportRoles\data\Queue.

Like any ESE database, the queue database uses log files to accept, track, and maintain data. To enhance performance, all message transactions are written first to log files and memory, and then to the database file. The checkpoint file tracks the transaction log entries that have been committed to the database. During an ordinary shutdown of the Microsoft Exchange Transport service, uncommitted database changes that are found in the transaction logs are committed to the database.

Circular logging is used for the queue database. This means that transaction logs that are older than the current checkpoint are immediately and automatically deleted. Therefore, the transaction logs can't be replayed for queue database recovery from backup.