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batch_handling

Since: Dev Builds Only

The functionality described in this section requires a dev build of KumoMTA. You can obtain a dev build by following the instructions in the Installation section.

SMTP messages can have an envelope that includes multiple recipients. Each recipient will receive a copy of the message. If multiple recipients share the same mailbox provider then it is advantageous from a bandwidth and efficiency perspective to relay that message to that provider as a single message with a list of multiple recipients, rather than sending one distinct copy per recipient.

The batch_handling option specifies how incoming multi-recipient transactions are split into outgoing batches.

It can have one of two values:

  • "BifurcateAlways" - this is the default and recommended setting for sender-focused deployments. Every incoming recipient is placed into a separate batch and tracked separately.
  • "BatchByDomain" - recipients with exactly the same domain portion are grouped together, resulting in one outgoing batch per unique domain.

If you have more advanced requirements around managing batching/splitting, then you can implement them via the smtp_server_split_transaction event handler.

kumo.start_esmtp_listener {
  batch_handling = 'BatchByDomain',
}