Hotmail, MSN and AOL (AOL you already know of) are cracking down on Spam, but, it seems particular to "spam" that comes from automated Content Managment Systems (like phpNuke, phpBB and Vbulletin). I've heard tons and tons of stuff from people and from people running hosting services that the above 3 are just completely banning emails to them that come from CMS type systems. I've been experiencing it for the last week or so on my own site, un-routable emails due to MSN, Hotmail and AOL filtering phpnuke and phpBB strings and now they've blocked the host I'm with
