Per the UK Guardian:
http://www.guardian.co.uk/elsewhere/journalist/story/0,7792,1126294,00.htmlQuoting:
Jewish activists opposing the Israeli government's policies face intimidation and harassment via email and on the internet.
...
Deborah Fink is a singer and music teacher living in London. She is also Jewish. Last month, out of the blue, she received a deluge of hateful emails - more than 150 in the space of a week. One came from a rabbi in New York, informing her: "Your soul, my dear, is petrified and lost." Another said, menacingly: "Hitler killed the wrong Jews."
Per the article, the emails and other forms of harassment are organized via several very viscious Web sites and email lists.