Bug#343348: kmail: Answer to a mail is send to a wrong address
> I use the term diffusion list but the diffusion address is similar to
> a normal one and I don't understand why kmail behaves differently
> with it.
A mail can have different headers, in particular one for where to post an
answer, an one that mailing lists use.
The behaviour of your kmail depends on those headers, so you should check
them and see if your bug is a bug or not.
At least, post them here -- but use the reply to all option, just to bet