Share via

Exchange Queues

  • Article

A queue is a temporary holding location for messages that are waiting to enter next stage of processing or delivery to a destination. Each queue represents a logical set of messages that the Exchange server processes in specific order. In Microsoft Exchange Server 2013, queues hold messages before, during and after delivery. Queues exist on Mailbox Servers and Edge Transport Server.

Type of Queues

The following types of queues are used in Exchange 2013.

Persistent Queues: Persistent Queues are queues that exist on every transport server in exchange organization.

  • Submission queue — Used by the categorizer to gather all messages that have to be resolved, routed, and processed by transport agents on the transport server. All messages that are received by a transport server enter processing in the Submission queue. On Mailbox servers, messages are submitted through a Receive connector, the Pickup or Replay directories, or the Mailbox Transport Submission service.
  • 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. Each transport server has only one Unreachable queue.
  • Poison message queue — Special queue that's used to isolate messages that are determined to be harmful to the Exchange 2013 system after a transport server or service failure. The messages may be genuinely harmful in their content and format.

Delivery queues: Delivery queues hold messages that are being delivered to any local or remote destinations by using SMTP. All messages are transmitted between Exchange servers by using SMTP. Non-SMTP destinations also use delivery queues if the destination is serviced by a Delivery Agent connector. Each delivery queue contains messages that are being routed to the same destination. It's practically inevitable that multiple delivery queues will exist on a transport server. Delivery queues are dynamically created when they're required and are automatically deleted when the queue is empty and the expiration time has passed. The queue expiration time is controlled by the QueueMaxIdleTime parameter on the Set-TransportService cmdlet. The default value is three minutes.

Shadow queues: Shadow queues hold redundant copies of a message while the message is in transit.

Shadow redundancy was introduced in Microsoft Exchange Server 2010 to provide redundant copies of messages before they're delivered to mailboxes. In Exchange 2010, shadow redundancy delayed deleting a message from the transport database on a transport server until the server verified the next hop in the message delivery path completed delivery. If the next hop failed before reporting successful delivery back to the transport server, the transport server resubmitted the message to that next hop. Exchange 2010 servers used the XSHADOW verb to advertise their shadow redundancy support. If an SMTP server didn't support shadow redundancy, Exchange 2010 used delayed acknowledgement based on a configured time interval on the Receive connector to make a redundant copy of the message.

Safety Net: Safety Net retains copies of messages that were successfully delivered by the transport server. Although it's not accessible by queue management tools, Safety Net is just another queue in the queue database.