kumomta/docs/reference/kumo/make_egress_path/enable_pipelining.md

enable_pipelining

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When set to true (the default is true), then kumo will use the SMTP PIPELINING extension when it is advertised by the remote host.

When set to false, then PIPELINING will not be used even if it is advertised.

You typically do want to use PIPELINING when available, because it reduces the protocol overhead and round-trips, resulting in lower latency sends per message.