Email not sending using DSO with MPM_Prefork:

/var/log/exim_mainlog shows:
unable to set gid=32009 or uid=32008 (euid=0): system filter
OR
unable to set gid= or uid=0 (euid=0): forcing real = effective

The reason is the UID/GID perfork can use is quite limited, and for some reason it wants to use very high IDs.

Our Fix:
Edit the file:
/etc/apache2/conf.modules.d/000_mod_mpm_prefork.conf

Set the option:

LimitUIDRange 0 50000

cPanel Fix: https://docs.cpanel.net/ea4/apache/apache-module-mpm-itk/
Edit the file:
/etc/apache2/conf.d/includes/pre_virtualhost_global.conf

Set the option:

<IfModule mpm_itk.c>
LimitUIDRange 0 4294496296
LimitGIDRange 0 4294496296
</IfModule>