Mailbombing is allowed in the UK - according to a stupid judge
That is what you get what judges without any knowledge of technology and security have to treat complex cases like these. Does no-one sends them to a course (one day a year ?)
A teenager accused of sending millions of e-mails to his employer has been cleared because UK laws do not make a crime of what he did
He ruled that crashing an e-mail server with five million messages did not break statutes in the 1990 act.http://news.bbc.co.uk/1/hi/technology/4402572.stm
more about mailbombing on http://www.securingit.tk
how to defend, well, be sure to have a mailrelayserver (sometimes your virusmailserver) before your mailserver where you can add rules so that you can delete all mails from a certain address before arriving or just sending them to another emailserver that drops them dead.
Turn off your auto-responders and set your resend and administrative posts and alerts as low as possible (for example once every 6 hours)
it is the easiest way to get a whole network down and the tools they are all over the undernet.