deferred_spool

Danger

Enabling this option may result in loss of accountability for messages. You should satisfy yourself that your system is able to recognize and deal with that scenario if/when it arises.

When set to true, incoming messages are retained in memory until after their first transient delivery failure.

This can have a dramatic impact on throughput by removing local storage I/O as a bottleneck, but introduces a risk of forgetting about those messages if the machine loses power or if the kumod process exits unexpectedly.

kumo.start_esmtp_listener {
  -- ..
  deferred_spool = false,
}