A. Service packs are a double-edged sword. On one hand, they always include necessary fixes and frequently contain desirable functionality as a bonus (e.g., the quite nice Mailbox Manager in SP3). However, each service pack seems to have its thorns (particularly, for some reason, SPs with even numbers) and SP3 is no exception. The Microsoft articles XGEN: List of Bugs Fixed in Exchange Server 5.5 Service Pack 3 (Part 1) and XGEN: List of Bugs Fixed in Exchange Server 5.5 Service Pack 3 (Part 2) do indeed point out some serious problems. Are the fixes worth the potential downside? In general, I advise administrators to test service packs on lab systems before installing them on production servers, especially if your lab-box configuration closely matches that of your production boxes. I also recommend keeping a close watch on Exchange Server forums, such as the one at the Swynk.com Web site, to find out what problems early adopters have run into. In the specific case of SP3, unless you're worried about one of the problems listed in the Microsoft articles mentioned above, go ahead and install it, but be sure to also install the post-SP3 hotfixes and run the Performance Optimizer after you do the install.