[Exim] owner.group of log files?

At our site, we like to have the exim log files be owned by a
group which our support department is member of, so that they can help
debug user email problems.

I have set the log directory to that group, and set the SetGID bit. I
have set all the current and archived files to that group. and set
group read permission.

Yet something (exim? exicylog?) keeps setting the current main or
reject log to group 'mail', which is the GID under which exim runs..

How the heck can I stop this, and force it to always be the gid I
want?

I grepped through spec.txt for 'log owner', 'permissions', etc.. and
couldnt find anything related.