I received an interesting challenge / request to speed up a clients server that runs the email marketing tool, Active Campaign.
One of the first things I did was move the Exim queue to SSDs.
Thereafter I pushed the following settings, to Exim.
root@mailserver ~ # cat /etc/exim4/conf.d/main/01_exim4-deliver_speedup
# THIS FILE IS AUTOMATICALLY DISTRIBUTED BY PUPPET. ANY CHANGES WILL BE
deliver_queue_load_max = 20
queue_only_load = 20
queue_run_max = 150
remote_max_parallel = 200
smtp_connect_backlog = 50
smtp_accept_max = 500
smtp_accept_queue_per_connection = 2000
On going live, we have identified that the client can deliver 90 000 newletters in under two and a half hours.