[JohnTHaller]

Most email providers block dynamic IP ranges because of the absolute torrent of spam that flowed from them before the block. Something like 90% of the world's spam used to come from hacked home PCs. Someone would install a hacked version of Photoshop, a "porn downloader", or get drive by installed and their PC would be under someone else's control and able to send out spam after spam after spam on an open port 25 from every ISP in the world. To counteract that, ISPs started blocking outgoing port 25 connections and most email server providers began blocking incoming connections from dynamic IP space. This cut down on spam dramatically.