Update Rollup 6 for HMC 4.5 is now available here http://support.microsoft.com/?id=979704. Note that this Rollup requires Exchange SP2, with Rollup 1.

Microsoft has released an update that is dated December 2009. This update fixes issues in the following components in Microsoft Solution for Hosted Messaging and Collaboration 4.5.

• Exchange 2007 Resource Manager Namespace
• Exchange 2007 Resource Manager Web Services
• Hosted Email 2007 Namespace
• Hosted Email 2007 Web Services
• Managed Email 2007 Namespace
• Managed Email 2007 Web Services
• Exchange 2007 Mobility Provider
• Exchange 2007 Provider
• Unified Messaging 2007 Provider

This article describes the following items about the update rollup:

• The issues that the update rollup fixes.
• The prerequisites for installing the update rollup.
• Whether you must restart the computer after you install the update rollup.
• Whether the update rollup is replaced by any other update rollup.
• Whether you must make any registry changes.
• The files that the update rollup contains

There is a statement in the KB article that is a little off and can be ignored safely, which is

 Additionally, this update also includes native 64-bit versions of the following components:

• Exchange 2007 Mobility Provider