If you use Exchange 5.5 or Exchange 2000 then you might want to consider your options as support policies will soon change.
As you probably know, Microsoft's mainstream support for Exchange 5.5 ended December 31, 2003 and extended support is available only until December 31, 2005 to those who purchased such support. After that time the product's life cycle will end and as such the company probably won't make any future hotfixes available.
Mainstream support for Exchange 2000 platforms ends December 31, 2005. You can opt to purchase an extended support agreement but you must make that purchase with the first 90 days of 2006, which extends support coverage until December 31, 2010. Without the extended coverage you probably won't get able to get access to hotfixes, including security-related hotfixes. More information is available in Microsoft's Support Policy Lifecycle FAQ.