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PMTA knowledge base›Metric§ 5.3

Queue depth

Queues
How much is waiting to be sent

Recipients, recipient domains and kilobytes in the queue.

🔴 The number of recipients also includes those deliberately unavailable for sending — deferred after a soft failure. A growing queue means either a jam or normal waiting, and this number alone cannot tell them apart.

It is also the multiplier for memory sizing: about two kilobytes for each recipient in the queue.

The main trap is taking a snapshot for a history. The metric describes the moment of polling. Everything that happened between two measurements and managed to end will not get into either of them. For events there is accounting, for state — these numbers.
What the Hub does. It collects it — node polling takes this metric from the status snapshot and stores it in its time series.

It is also the multiplier for memory sizing: about two kilobytes for each recipient in the queue.

Recommended values

What to compare with:

Value Compared with
current value the usual one for this node, not the neighboring one
peak the ceiling, if there is one
growth the growth rate, not the absolute number

Fleet nodes are not directly comparable: they have different hardware, different addresses and a different mix of receivers. It makes sense to compare a node with itself over time.

The comparison advice is ours. From the manual here come the metric’s composition and its units of measurement.
Example configuration
# the metric comes in the status snapshot
curl -s "http://127.0.0.1:8080/status?format=json"pmta show status
The highlighted line returns the snapshot as a structure — that is how the Hub’s polling reads it. The line below prints the same thing in the console.
yellow — — directive names, blue — blocks. They are highlighted the same way in the text of the card.
Compatibility
PowerMTA 6.0
described in the manual of this version · platform: any
The vendor does not state the first version for every directive — there is no note here. The name also appears in the 5.0 edition of the manual.
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Related entries

3 relations

Depends on

Spool capacityThe depth is limited by the spool capacity in recipients. § 5.3

Works together with

QueuesHere is the node total; there the same mail is broken down by pairs. § 5.4

Applies to

How a message leaves the nodeThe depth breakdown follows the delivery methods: when messages are discarded, the queue does not grow at all. § 5.3

Referenced from

ConnectionsQueue depth without connections is the sign of a node that is not sending, rather than busy. inferenceHand a message to a webhook instead of a receiverAn unavailable handler accumulates a queue rather than losing messages — and is visible precisely by its depth. inference

How to check on the node
The metric is viewed together with the neighboring ones — on its own any number means little:
pmta show status
pmta show queues --sort=rcpt --maxitems=20
The first prints all node metrics at once. The second breaks down by queue what the first gives as one number — and that is usually where you can see which queue made it up.
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What else to read

3 articles

◇What the node tells about itself and through which channelsSeven observation surfaces: the web monitor, the console in four formats, the log, accounting, accounting search, metrics and export to external analytics.◇The watchdog that restarts the engineKeeps an eye on the engine and brings it back up after a crash by itself, sending a crash report to the postmaster address.◇What makes a virtual machine riskyRunning on a virtual machine is possible, but the manual does not recommend it without sufficient hardware and properly tuned hypervisor settings: under load the node may stop responding.

In the Hub
Cost of a mistake
safe: a wrong value breaks nothing
Scenarios
Emergency stop · Untangle a stuck queue