[Exim] Problems with accentuated chars

I recently migrated my mail serveur from Exim 3 to Exim 4. Everything seems
to work fine, except that *some* mails with accentuated chars seems to be
"converted" and all the accents are changed to garbage. I don't have any
rule to change the content of the messages (no spam detection or such), so
I'm puzzled...
Any idea on how I can investiguate this?

TIA

Greg