I don't know about filters, but I had the same 'problem' with Kmail ... also Kmail would 'crash', or shall I say, close completely from the Kicker. Everytime I reopend, there would be another email.
I had a friend try also and he had the same 'problem' but his preference was to use another emailer anyway.
Mandriva was not very stable on my system ... I now use Suse 10