[exim] does exim run as root or not?

Hi,

also after reading through chapter 51, I'm not sure what's up here.

On my system (Debian Sarge) the exim (4.60) binary is as following:
-rwsr-xr-x 1 root root 811224 2006-01-22 11:53 /usr/sbin/exim4

It's started on system startup and its parent is user 1. ps says it runs
as user Debian-exim and group Debian-exim (euid, egid, ruid and rgid are
102 too).

And regardless of this, it can write in directories with
drwx------ normaluser users
and call e.g. maildrop with uid=1000 (normaluser) and gid=100 (users).

How does this work if it's not root?

Bye,
Jürgen