Hosted Messaging and Collaboration Knowledge Base Articles
This page used to provide just the HMC related updates but now it has been updated to provide the specific product service packs, rollup updates, hotfixes that have been tested by the HMC product group. With this, I will no longer provide any quarterly list of updates on what hotfixes have been tested like I used to. :) Here is the link,
For example, you can see the Update Rollup 9 for Exchange Server 2007 Service Pack 1 (KB970162) have been tested for HMC 4.x.