Installed debian from DVD. And now I see exim4 is running owned by UID 107. There is no user 107 in my /etc/passwd.

Same problem on another system (owned by UID 101), so I suspect this is a debian problem...?

Running squeeze on both.

So under debian, who should own the mail system?

This is what I have installed:

# dpkg -l | grep exim
ii  exim4                              4.72-6+squeeze2              metapackage to ease Exim MTA (v4) installation
ii  exim4-base                         4.72-6+squeeze2              support files for all Exim MTA (v4) packages
ii  exim4-config                       4.72-6+squeeze2              configuration for the Exim MTA (v4)
ii  exim4-daemon-light                 4.72-6+squeeze2              lightweight Exim MTA (v4) daemon

The binary itself is owned by root:

-rwsr-xr-x 1 root root 758852 May 12  2011 /usr/sbin/exim4