I have a dedicated customers that I have convinced to migrate to cPanel from other solutions that used qmail, such as Plesk and non panel server configs.

These people all love cPanel but are dissatisfied with the performance of Exim.

One person for example would mass mail his members (member site, opt-in list) of about 40k users. And according to him it would take approx 2 -4 hrs to email 40k+ users with qmail, but on the cPanel box with Exim the list is taking 2 days to process.

He doesn't expect miracles but would like to see a performance increase. Is there anything I can do to change how large of batches Exim uses to send emails, or to optimize it to run better on the server?

Thanks