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Why is KumoMTA Accepting Connections From Systems Not Listed in relay_hosts?

Let's say you have the following configured for your listeners:

kumo.start_esmtp_listener {
  listen = '',
  relay_hosts = { '', '' },

And the following configured for your `listener_domains`` helper:

relay_to = false
log_oob = true
log_arf = true

As you operate your server, you may be concerned to discover in your logs that messages are bring accepted from hosts not listed in your relay_hosts list.

  "type": "Reception",
  "id": "ebaaab5e9e7b11eeb206960002ccea16",
  "sender": "",
  "recipient": "",
  "queue": "",
  "site": "",
  "size": 594,
  "response": {
    "code": 250,
    "enhanced_code": null,
    "content": "",
    "command": null
  "peer_address": {
    "name": "WIN-CLJ1B0GQ6JP",
    "addr": ""
  "timestamp": 1702996556,
  "created": 1702996556,
  "num_attempts": 0,
  "bounce_classification": "Uncategorized",
  "egress_pool": null,
  "egress_source": null,
  "feedback_report": null,
  "meta": {},
  "headers": {
    "Subject": "test smtp"
  "delivery_protocol": null,
  "reception_protocol": "ESMTP",
  "nodeid": "dd2b41fd-78f0-4105-8cd9-01ac7114cada"

This occurs because of the log_oob and log_arf options being enabled for * in the configuration.

Whenever any of the relay_to, log_oob, or log_arf options are set to true, KumoMTA cannot immediately reject connections from hosts not listed in relay_hosts because it needs to see whether the remote host is sending messages to one of the domains listed in the listener_domains helper configuration.

After a RCPT TO command is received, KumoMTA will check whether the destination domain is listed in the listener_domain config and if so, will receive the message and act accordingly. This can result is messages being received that may or may not be queued after being processed as an OOB and/or ARF message, depending on whether relay_to is set to true.