Quote:
"�(1) A person is guilty of an offence if�
(a) he causes a computer to perform any function with intent to secure access to any program or data held in any computer;
(b) the access he intends to secure is unauthorised; and
(c) he knows at the time when he causes the computer to perform the function that that is the case"
Source:
Computer Misuse Act 1990, Section 1 (which also specifies a maximum penalty of a �5000 fine and/or 6 months in prison):
http://www.opsi.gov.uk/acts/acts1990/Ukpga_199 00018_en_2.htm#mdiv1
('Access to data in any computer' would certainly include accessing another person's e-mail on the server of their account provider).
Chris