OT: Spam Problems

[Errol Mangwiro]

Hi,

Recently I started receiving bounces from mail purporting to having been sent from addresses in my domain. But the addresses don't exist. So I thought that someone was faking the sender header and sending spam. I added SPF and domain-key records to try to combat this. However, either hotmail and yahoo don't check these or they ignore them because I'm still getting spammed. 

Does anyone know of a way I can tighten fake sender policies & prevent this from occuring again?

Thanks
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"No man is an island, entire of itself; every man is a piece of the continent, a part of the main; if a clod be washed away by the sea, the World is less, as well as if a promontory were, as well as if a manor of thy friends or of thine own were; any man's death diminishes me, because I am involved in mankind; and therefore never send to know for whom the bell tolls; it tolls for thee."