SPEWS is a list of areas on the Internet which several system administrators, ISP postmasters, and other service providers have assembled and use to deny email and in some cases, all network traffic from.
It is also a load of shit in some cases, putting several sites on its list that don't deserve to be on it. Including but not limited to deviantart.com and somethingawful.com
Hotmail uses SPEWS, and by doing so, dis-allows users on the sites which are blacklisted to recieve any mail from those sites.
Spews sure doesn't take into account that some of the sites on its list are undeserving of being placed upon it.