Ok, after a bit more investigation, I finally conclude that it's a problem from courier-imap. Indeed, I said thunderbird and mutt didn't have the problem, but in fact, they just doesn't report the problem. They have the problem like kmail, but without displaying any error message.
So, you can close this bug (sorry for disturbing). Plus simple, plus fiable, plus rapide : découvrez le nouveau Caramail - http://www.caramail.lycos.fr
