`kumo.on('smtp_client_rewrite_delivery_status', function(response, domain, tenant, campaign, routing_domain))`

Since: Version 2023.11.28-b5252a41

The functionality described in this section requires version 2023.11.28-b5252a41 of KumoMTA, or a more recent version.

This event is triggered by the SMTP client if a message is rejected by a destination SMTP server.

Rejected means one of:

MAIL FROM or RCPT TO got a non-2xx response
DATA got a non-354 response
The final CRLF.CRLF got a non-250 response

In that circumstance, the SMTP response is formatted into a single line (replacing CRLF with the literal \r\n sequence) and the queue name parameters are extracted in order to call the smtp_client_rewrite_delivery_status event.

The purpose of the event is to enable you to make a policy decision to optionally rewrite the status code. For example, you may wish to treat a full mailbox as a permanent failure if the nature of the message is some kind of bulk notification that won't be missed.

The event can return the its own version of the SMTP status code which will be used when considering what to do with the message.

For instance, if the event returns a 500 code when the original code was a 400 code, the message will be logged as a Bounce and then removed from the spool.

If the event returns null, or the same value as the original response, then the message is processed as normal.

Otherwise, the response string will have (kumomta: status was rewritten from 400 -> 500) appended to it, so that it is clear from the logs that a rewrite occurred.

This example shows how to build up a mapping table from a json file with contents like this:

{
  "4\\.2\\.1 <.+>: Recipient address rejected: this mailbox is inactive and has been disabled": 500,
  "4\\.2\\.2 The email account that you tried to reach is over quota\\.": 500
}

-- Compile a classifier from the json file; refresh it it every 5 minutes
local get_dsn_classifier = kumo.memoize(function()
  local data = kumo.json_load '/tmp/dsn_rewrite.json'
  return kumo.regex_set_map.new(data)
end, {
  name = 'dsn_rewrite',
  ttl = '5 minutes',
  capacity = 1,
})

-- This example ignores the queue name parameters, but you could get more
-- sophisticated and use those to define rules on a per-domain/tenant/campaign basis
-- if required
kumo.on(
  'smtp_client_rewrite_delivery_status',
  function(response, domain, tenant, campaign, routing_domain)
    local map = get_dsn_classifier()
    -- Match the classifier against the response.
    -- This will return the rewritten code if any, or null otherwise.
    -- We can simply return the result of the lookup directly.
    return map[response]
  end
)