[Exim] Auth - Exim and /etc/shadow file

Hi !

I use MTA Exim under Debian. Exim use passwords from /etc/shadow.
For default settings:

$>  ls -l /etc/shadow
-rw-r-----    1 root     shadow       1072 Sep 14 20:45 /etc/shadow

Exim don't send messages.

I add 'mail' user to 'shadow' group. Exim also don't send messages.

$> cat /etc/group | grep shadow
shadow:x:42:mail

But when I change group 'shadow' file on 'mail'

$>  ls -l /etc/shadow
-rw-r-----    1 root     mail       1072 Sep 14 20:45 /etc/shadow
-------------------------------------------------------------------

Exim works ok, but this not good idea.

How to set this correctly ?
Thaks for your help.

Slawek