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Configuring Bounce Classification

By default, the logs will contain extensive information on the responses provided by a remote host during a delivery attempt:

// The response from the peer, if applicable
"response": {
    // the SMTP status code
    "code": 250,

    // The ENHANCEDSTATUSCODE portion of the response parsed
    // out into individual fields.
    // This one is from a "2.0.0" status code
    "enhanced_code": {
        "class": 2,
        "subject": 0,
        "detail": 0,

    // the remainder of the response content
    "content": "OK ids=8a5475ccbbc611eda12250ebf67f93bd",

    // the SMTP command verb to which the response was made.
    // eg: "MAIL FROM", "RCPT TO" etc. "." isn't really a command
    // but is used to represent the response to the final ".:
    // we send to indicate the end of the message payload.
    "command": "."

This information includes the IANA Status Codes provided by the remote host, but there are a large number of codes that can be interpreted in a variety of ways, and many mailbox providers use status codes differently.

To make it easier to handle bounces, the Bounce Classifier can be configured:

kumo.on('init', function()
  kumo.configure_local_logs {
    log_dir = '/var/log/kumomta',
  kumo.configure_bounce_classifier {
    files = {

Once configured, the Bounce Classifier will populate the bounce_classification field in the logs with the applicable category.

An example of classification rules:

InvalidRecipient = [
  "^(451|550) [45]\\.1\\.[1234] ",
  "^45[02] [45]\\.2\\.4 ", # Mailing list expansion
  "^5\\d{2} [45]\\.7\\.17 ", # RRVS: Mailbox owner has changed
BadDomain = [
  "^(451|550) [45]\\.1\\.10 ", # NULL MX
  "^5\\d{2} [45]\\.7\\.18 ", # RRVS: domain owner has changed

Users can create their own classification rules file by copying the default file, editing it, and adding the path to their custom rules file to the files option in the kumo.configure_bounce_classifier function call. Each defined rules file will be merged into the full ruleset.

For additional information, see the reference manual page on bounce classification.

Configuring Out-Of-Band Bounce Processing

Not all bounces occur while the server is connected to the remote host. It is not uncommon for a remote host to accept a message, perform further processing, and then determine that the message should be rejected. This can be because of user validation, AntiVirus/AntiSpam processing, or other post-connection logic. When a remote host rejects a message after accepting it, RFCs require that a Message Disposition Notification (MDN) message be sent to the return path address of the message specified in the MAIL FROM command during the relay session. See for more information.

The KumoMTA server can process these MDN messages, but must be configured to know which domains are candidates for OOB bounce processing. This is most simply accomplished using the listener domains policy helper, with a snippet like this:

# accept and log OOB messages send to
log_oob = true

OOB Message Disposition After Processing

For most use cases, the desired outcome after a DSN message is processed is to discard the message, but in some cases it can be desirable to forward the message for further processing or storage, especially during testing and migration.

To queue a message after processing, add relay_to = true to the listener domain configuration:

# accept and log OOB messages send to
log_oob = true
relay_to = true

In addition, it should be noted that the MX record for your domain will still be pointed at the KumoMTA instance, which means that in order to avoid a mail loop you will need to configure routing for the domain to specify where the message should be relayed to from the KumoMTA instance:

kumo.on('smtp_server_message_received', function(msg)
  if msg:recipient().domain == '' then
    -- Re-route the message to the intended destination
    msg:set_meta('routing_domain', '[]')

    -- In earlier versions of KumoMTA, you need to set the queue
    -- this way instead
    -- msg:set_meta('queue', '[]')