How do I create an always-suspended queue?

Some sites employ a catch-all queue for messages that didn't get categorized and queued into an expected queue. In the Momentum (AKA Ecelerity) MTA they might do this by defining a binding or binding group that has delivery permanently suspended.

The equivalent concept in KumoMTA is to define an Egress Pool that has no sources, and associate the always-suspended queue with that pool.

That will cause messages that are queued to it to experience a transient failure with a disposition of this form:

451 4.4.4 no non-zero-weighted sources available for QUEUE_NAME

Messages will be subject to the usual retry schedule, which you can of course configure for your queue.

To configure this using the queues and sources helpers:

• Add the following to your queues.toml:

TOMLJSON

[queue."always-suspended"]
egress_pool = "always-suspended-pool"

{
  "queue": {
    "always-suspended": {
      "egress_pool": "always-suspended-pool",
    },
  },
}

• Add the following to your sources.toml:

TOMLJSON

[pool."always-suspended-pool"]

{
  "pool": {
    "always-suspended-pool": {
    },
  },
}

• Then in your policy, you can put messages in that queue explicitly:

kumo.on('smtp_server_message_received', function(msg)
  if some_condition then
    -- Explicitly assign the message to the always-suspended queue.
    -- Its envelope, tenant and campaign will be ignored as the queue
    -- meta value takes precedence
    msg:set_meta('queue', 'always-suspended')
  end
end)