Update your Exchange servers as soon as possible
Microsoft has just released security update for all the current supported Exchange versions. Exchange 2000 made it just in time, since its extended support ends on the 11th of January 2011.
So please update your Exchange Servers as soon as possible to avoid a particular denial of service attack.
More information and links to the updates can be found on the Micrsoft Exchange team blog site: http://msexchangeteam.com/archive/2010/04/13/454610.aspx