Migrating from a very old Debian install to a newer VPS with a more recent version of Apache and the mpm-itk mod, I was having problems with sending mail using the standard PHP mail() call (first seen when the WP contact form I was using was throwing a “Sorry, email message could not be delivered” error).
exim4 log reported the following:
unable to set gid=33 or uid=0 (euid=0): forcing real = effective
This thread contained a post indicating the problem was the LimitGIDRange/LimitUIDRange options; it seems if these are not specified there are some defaults (perhaps with very low values, or perhaps it’s just that if it’s not set it will not work at all) that need to be overridden.
Defining these values in the global Apache configuration fixes it.