Table E.1 charts the different releases of Exchange since the product was first released, including all the service packs that have been issued for each version. The column on the left lists the build number for the release. Microsoft builds Exchange each night, so the build number is forever changing. Because there are so many builds, it's a good idea to know how to correlate the build number against versions. It is always possible that a software component from one version or service pack isn't properly updated on a system, especially if problems occur during an installation. More commonly, you can get version skew through the application of hot fixes, or if administrators copy files from one server to another when the two servers aren't running the same version. This sounds like a very strange thing to do, but it does happen.

The figures to the left of the decimal point indicate the major build number. Those to the right are the minor build number and reflect minor changes, perhaps to a specific component that doesn't necessitate a complete new build. If you install a hot fix from Microsoft, it is likely that the minor build number will change. If the updated code comes from a future service pack, the major build number may change. Never apply a hot fix unless you are advised to do so by Microsoft product support, either explicitly or through instructions contained in a Knowledge Base article. The interaction between the different parts of Exchange...