You're wise to be wary of any unsolicited e-mails but this one could be genuine.
Gordon Brown has increased air passenger duty, with effect from February. For most flights, the tax rises from �5 to �10:
http://news.bbc.co.uk/1/hi/uk_politics/6213328 .stm
easyJet's terms and conditions include the following:
"Any taxes or charges imposed by Government or other authority, including the operator of an airport, which we are obliged to collect from you or pay in respect of you and your Flight may be payable by you to us in addition to the Fare. At the time you make a reservation you will be advised of the total amount of such taxes and charges not included in the Fare and payable by you. Such taxes and charges imposed on air travel are constantly changing and can be imposed after the date that your reservation has been confirmed. If any such tax or charge is introduced or increased after the reservation was made you will be obliged to pay it prior to departure":
http://www.easyjet.com/EN/Book/conditions.html
So it would seem reasonable to expect that easyJet customers can expect to receive a demand for payment.
If the e-mail you've received quotes your travel details (or your booking reference number) it's almost certainly genuine. (A fraudster could only get these details by hacking easyJet's booking system or by breaking through your firewall to find the data on your own PC). If you're still not sure, phone easyJet Customer Services: 0871 244 2366.
Chris