If you've always deployed the newest versions of Microsoft products, you might not be familiar with the distinctions the company makes in its support policies for products that have reached their end-of-support dates. Before you decide whether to stick with your Exchange 2000 Server or Exchange Server 5.5 installation instead of moving to Exchange Server 2003, be sure you understand the two support levels that apply to those older products so that you'll know what you will—or won't—be giving up by staying with them.
Mainstream support is what you get for current products and comprises all the support services available through Microsoft Product Support Services (PSS), including free hotfixes and bug resolutions, per-incident and per-hour support, and support through contracts such as Microsoft Premier Support. Microsoft commits to a minimum of 5 years of mainstream support for business server products, although it can provide more support at its discretion (as it did for Exchange 5.5).
Extended support becomes available for a limited period, beginning the day after mainstream support ends. You must pay for extended support. Microsoft has a good track record of providing free security hotfixes for products that are in extended support, but you'll have to pay for hotfixes for other types of problems. You might be able to buy a custom support contract to cover a given product after extended support has ended. I don't know of anyone who has purchased such a contract for Exchange, but Microsoft has made custom contracts available to Windows NT customers.
On December 31, 2005, Exchange 5.5's extended support period ended and Exchange 2000 moved to extended support. These changes weren't unexpected, as Microsoft had been telling customers about them since August 2003. However, as with many milestone dates, not everyone who was affected took steps to upgrade or migrate to a supported version of Exchange.