And the destination_rat_delay might delay with smtp_destination_concurrency_limit, from the same comment:
smtp_destination_concurrency_limit has no effect when you turn on smtp_destination_rate_delay. It specifies a delay BETWEEN deliveries, meaning the deliveries cannot be in parallel.
Not sure these do anything:
via:
And the destination_rat_delay might delay with
smtp_destination_concurrency_limit
, from the same comment: