Skip to content

Queues, Sources, Pools

Messages are assigned to a Scheduled Queue upon reception. The system can support arbitrarily named queues but the convention is to construct the queue name from some metadata associated with the message:

  • tenant - some kind of sender or customer identifier or identity derived either from the message itself or authentication of the injection session.
  • campaign - a sender-provided label that can be used to logically group a set of related messages, perhaps generated from the same campaign.
  • destination domain - the site where the email will be routed
  • routing_domain - (Since: Version 2023.08.22-4d895015) overrides the destination domain for routing purposes.

These three pieces of information are combined to produce the name of the queue in the form campaign:tenant@domain. You don't need to explicitly assign a campaign or a tenant. The queue name will be formed based on what is set:

tenant set? campaign set? routing_domain set? Resulting queue name
yes yes no campaign:tenant@domain
yes yes yes campaign:tenant@domain!routing_domain
yes no no tenant@domain
yes no yes tenant@domain!routing_domain
no yes no campaign:@domain
no yes yes campaign:@domain!routing_domain
no no no domain
no no yes domain!routing_domain

At reception, in your smtp_server_message_received event, you have the opportunity to set tenant and campaign meta values, if desired. By default, the queue associated with the message will be formed from those values as described above.

You may also explicitly set the queue meta value directly.

This is useful in some special cases. For example, there is a special null queue that simply discards any messages assigned to it, which is useful when implementing a sink for testing purposes:

kumo.on('smtp_server_message_received', function(msg)
  -- Accept and discard all messages
  msg:set_meta('queue', 'null')

Egress Sources and Pools

Once assigned to a Scheduled Queue, the system will attempt to deliver it. The first stage is to resolve the configuration for the queue by triggering the get_queue_config event.

One of the queue configuration parameters is the name of the Egress Pool which should be used when delivering messages.

A pool is a group of Egress Sources. Each source can be associated with a local IP address or configured to use an alternative destination port (and in the future: use the HAProxy protocol) as the source of the outgoing traffic to the destination domain.

kumo.on('init', function()
  kumo.define_egress_source {
    name = 'ip-1',
    source_address = '',
  kumo.define_egress_source {
    name = 'ip-2',
    source_address = '',
  kumo.define_egress_source {
    name = 'ip-3',
    source_address = '',

  kumo.define_egress_pool {
    name = 'pool1',
    entries = {
      { name = 'ip-1' },
  kumo.define_egress_pool {
    name = 'pool2',
    entries = {
      { name = 'ip-2', weight = 2 },
      -- we're warming up ip-3, so use it less frequently than ip-2
      { name = 'ip-3', weight = 1 },

local TENANT_TO_POOL = {
  ['tenant-1'] = 'pool1',
  ['tenant-2'] = 'pool2',
  ['tenant-3'] = 'pool1',

  function(domain_name, tenant, campaign, routing_domain)
    return kumo.make_queue_config {
      egress_pool = TENANT_TO_POOL[tenant],

When a message in a given queue is ready for delivery, it will use the configured pool to set up an Egress Path for the message to reach its destination. It will pick a source using weighted round robin from the entries in the pool, and call your get_egress_path_config event to resolve options to control how it will do that.

The parameters to that event are:

  • domain - the destination domain name
  • egress_source - the selected egress source
  • site_name - a string deterministically derived from the MX record for domain.

The site_name concept allows managing traffic more effectively for domains that have several or even a great many aliases. Rather than treating each domain as a separate destination, the traffic can be grouped together and managed more closely to how the receiving site sees it: as one source.

With that in mind, the egress path is internally identified by the combination of the egress_source and the site_name.

-- Build a mapping from a domain name to the "site name"
-- You'll likely want this to list the major domains for which there are
-- several aliases. You only need populate entries for the big aliased
-- domains; you needn't specify any of the aliases here.
local SITE_OF = {}
for _, domain in ipairs { '', '' } do
  SITE_OF[domain] = kumo.dns.lookup_mx(domain).site_name
-- SITE_OF is now equivalent to:
-- local SITE_OF = {
--   [""] = "(alt1|alt2|alt3|alt4)?",
--   [""] = "(mta5|mta6|mta7)"
-- }

-- This table is keyed by either site name or domain name.
-- Site names are looked up first, then domain names.
  [SITE_OF['']] = {
    -- appropriate general configuration when communicating
    -- with any domain that shares's MXs
    max_connection_rate = '1000/hr',
  [''] = {
    -- appropriate general configuration when communicating
    -- specifically with
    max_connection_rate = '10/hr',

-- This table is keyed by the tuple of (site_name, source) or (domain, source).
-- Site names are looked up first, then domain names.
-- Values override/overlay those in SHAPE_BY_DOMAIN.
  [{ SITE_OF[''], 'ip-1' }] = {
    -- appropriate configuration for your source when communicating
    -- with any domain that shares's MXs
    max_message_rate = '1000/hr',

function merge_into(src, dest)
  for k, v in pairs(src) do
    dest[k] = v

kumo.on('get_egress_path_config', function(domain, egress_source, site_name)
  -- resolve parameters first based on the site, if any,
  -- then based on the domain, if any,
  -- otherwise use the system defaults
  local domain_params = SHAPE_BY_DOMAIN[site_name]
    or SHAPE_BY_DOMAIN[domain]
    or {}
  local source_params = SHAPE_BY_SOURCE[{ site_name, egress_source }]
    or SHAPE_BY_SOURCE[{ domain, egress_source }]
    or {}
  -- compose the source params over the domain params
  local params = {}
  merge_into(domain_params, params)
  merge_into(source_params, params)
  return kumo.make_egress_path(params)

Message Flow

graph TD
   SQ["Scheduled Queue:"]
   SMTPL["ESMTP Listener"]
   HTTPI["Injection API"]
   RQ1["Ready Queue:>MX("]
   RQ2["Ready Queue:>MX("]
   POOL["egress pool"]
   MAINT["Queue Maintainer"]

   SMTPL --> SQ
   HTTPI --> SQ
   SQ --> MAINT
   IP1 --> POOL
   IP2 --> POOL
   POOL -- per tenant:domain config --> MAINT
   MAINT -- throttle per tenant:domain config --> RQ1
   MAINT -- throttle per tenant:domain config --> RQ2
   RQ1 -- throttle per source:domain config\nconnect via --> DESTSITE
   RQ2 -- throttle per source:domain config\nconnect via --> DESTSITE

Last update: 2023-10-18
Created: 2023-03-07