soft_bounce_drain_rate — how many soft bounces to place into the mail queues in a single scheduler iteration
soft_bounce_drain_rate = 100
When async_bounce_rendering is enabled soft bounces are queued internally. This option defines how many are pulled from the internal queue and placed into the mail queues in a single scheduler iteration.
The default value for this option is
soft_bounce_drain_rate is valid in the global scope.