As of October 11, 2011, Exchange 2010 RTM (that is, without any Service Packs) is losing support from Microsoft. Time to make plans for updating to the latest Service Pack and Update Rollup.
The announcement from the Microsoft Exchange blog can be found here:
Microsoft has released their latest Update Rollup for Exchange 2010 SP1. You can read about what is in the release at the Microsoft Exchange blog here:
Given that the last couple Update Rollup releases have been pulled back and re-released, you may want to consider taking a conservative approach to upgrading, only doing so if you have an issue addressed specifically by the rollup. We do, however, encourage you to stay up on the latest update rollups in general, after they have been proven in the field for a while.
Microsoft has fixed the issue that caused them to pull Rollup Update 4. You can find details of exactly what the issue was here:
Microsoft has taken down Exchange 2010 SP1 RU4. You can see the details here.