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The queue manager is a delivery agent scheduler which ensures fast and fair delivery of mail to all destinations within designated resource limits. By default, the active queue accommodates a maximum of 20000 messages. The messages to the active queue are from the incoming queue where new mails arrive and the deferred queue where old mails are retried. The messages in the active queue are ready to be sent, but are not necessarily in the process of being sent. In reality, the active queue is a set of data structures within the memory of the queue manager process. The envelope information for messages in the active queue is managed in memory, allowing the queue manager to do global scheduling, allocating available delivery agent processes to an appropriate message in the active queue. Within the active queue, the messages are broken up into groups of recipients that share the same transport/nexthop combination; the group size is capped by the transport's recipient concurrency limit. Multiple recipient groups (from one or more messages) are queued for delivery grouped by transport/nexthop combination. An active queue will be empty only when a queue manager marks a destination as dead (this may be due to the destination being down) and defers all the mails assigned to that destination to the deferred queue. An active queue may be overflowing if the destination is slow or if there is a problem in the destination which has caused excessive arrival of mails leading to a sudden surge in the active queue as well as congesting the destination. A destination may also be congested if the entire deferred queue undergoes an unwarranted flush. When messages reside in the active queue for a prolonged period, then the active queue may grow indefinitely and may not hold the space to accommodate all the messages. This will lead to a drastic degradation of the performance of the Postfix mail server. To avoid such unpleasant degradation and to keep track of the performance of the Postfix mail server, it is necessary to continuously track the messages in the active queue. The ActiveQueueTest helps administrators in this regard!

This test periodically monitors the active queue of the target Postfix mail server and reports the total size of the queue as well as the split up of the message count in terms of time duration i.e., the number of messages that were in the queue for a specified time duration.

Measurement	Description	Measurement Unit	Interpretation
Queue_size	Indicates the total size of the queue.	Number	Ideally, a low value is desired for this measure. An unusually high number of messages in the queue is indicative of any problem with the corresponding queue or its end points. The Detailed diagnosis of this measure lists the message count for each domain to which the mails are destined to.
Less_than_5_mins	Indicates the number of messages that were in the queue for less than 5 minutes.	Number	Under normal conditions, the active queue should be empty i.e., the mail should leave the system to the destination as quickly as it comes in.
Between_5_to_10_mins	Indicates the number of messages that were in the queue for a time duration between 5 to 10 minutes.	Number	Under normal conditions, the active queue should be empty i.e., the mail should leave the system to the destination as quickly as it comes in.
Between_10_to_20_mins	Indicates the number of messages that were in the queue for a time duration between 10 to 20 minutes.	Number	Under normal conditions, the active queue should be empty i.e., the mail should leave the system to the destination as quickly as it comes in.
Between_20_to_40_mins	Indicates the number of messages that were in the queue for a time duration between 20 to 40 minutes.	Number	Under normal conditions, the active queue should be empty i.e., the mail should leave the system to the destination as quickly as it comes in.
Between_40_to_80_mins	Indicates the number of messages that were in the queue for a time duration between 40 to 80 minutes.	Number	Under normal conditions, the active queue should be empty i.e., the mail should leave the system to the destination as quickly as it comes in.
Between_80_to_160_mins	Indicates the number of messages that were in the queue for a time duration between 80 to 160 minutes.	Number	Under normal conditions, the active queue should be empty i.e., the mail should leave the system to the destination as quickly as it comes in.
Between_160_to_320_mins	Indicates the number of messages that were in the queue for a time duration between 160 to 320 minutes.	Number	Under normal conditions, the active queue should be empty i.e., the mail should leave the system to the destination as quickly as it comes in.
Between_320_to_640_mins	Indicates the number of messages that were in the queue for a time duration between 320 to 640 minutes.	Number	Under normal conditions, the active queue should be empty i.e., the mail should leave the system to the destination as quickly as it comes in.
Between_640_to_1280_mins	Indicates the number of messages that were in the queue for a time duration between 640 to 1280 minutes.	Number	Under normal conditions, the active queue should be empty i.e., the mail should leave the system to the destination as quickly as it comes in.
More_than_1280_mins	Indicates the number of messages that were in the queue for more than 1280 minutes.	Number	Under normal conditions, the active queue should be empty i.e., the mail should leave the system to the destination as quickly as it comes in. An unusually high value of this measure may indicate that the active queue is congested. Administrators should therefore investigate the real reason for congestion and should take necessary steps to send the mails to the deferred queue or should set a rule intimating the users to try sending mails to the problematic domain later. Some of the possible reasons for congestion may be either the unavailability of the domain to which the mails are destined to or the domain may be slow.