This server was experiencing loads of up to 80 and maxing out the RAM and kmemsize on a CPanel VPS. There were literally dozens if not hundreds of exim processes. I have no idea why exim has such a design that would allow it to consume this much CPU and RAM. Any normal MTA should not be spawning so many processes, it should be processing them in sequence and if it is going to spawn hundreds of processes in response to a large volume of mail, it's better to have a delayed delivery than a crash etc..
The following settings in exim.conf are what fixed it (and should be the default to avoid a DOS effect), I never observed more than two exim proceses or a load about 1.57 after setting that and restarting Exim.
The true culprit of the high load were thousands of e-mails being sent without a recipient/to field from a form based contact script though.
deliver_queue_load_max = 3
queue_only = true
queue_only_load = 3
queue_run_in_order = true
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