[exim] Mail in UTF-8

Hi,

I'm a new user on this ml...
I have a problem with some applications like cron or logcheck. They send mail without specifying the hearders.
I don't know exactly how exim works, but I'd like to force UTF-8.
I've tried this :
mail -a "Content-Type: text/plain; charset=UTF-8" toto@???
and it is ok.
But is there any configuration file where I can say : use this header ?

Thank you very much (I hope I'm not off topic ;-))).

David.