From ComputerWeekly:
A judge has ruled that denial of service attacks are not illegal under the UK’s outdated Computer Misuse Act.
[…]
The youth had been accused of sending five million e-mails to his ex-boss as part of a DoS attack that crashed the company’s e-mail server.
He was charged under section 3 of the 1990 act, which covers unauthorised data modification and system tampering.
His defence argued that sending a flood of unsolicited e-mails did not cause unauthorised data modification or tamper with systems, as an e-mail server was there for the purpose of receiving e-mails.