How to speed up exim4?

Question

Mail server

The task is to send letters as quickly as possible.
We have a database of 400k legally collected addresses, people who have signed up themselves, so this is not spam.
I would like to send 400k letters in 1-2 hours.
Does anyone have recipes on how to quickly send so many letters from 1 server through exim4?
1. spool was stuffed into memory
2. parallel sending was turned
on in general, all the recipes from the Internet were tried, but it has about 15-20 letters per second, more is needed.
In terms of resources, we do not run into either the network or the disks or the processor.

Answer 1

Increase
queue_run_max
and remote_max_parallel (if one email is sent to a large number of addresses)