Microsoft released a technical document, HMC 4.0 - Scalability Test Report, that is the result from several scalability tests performed by Microsoft Enterprise Engineering Center (EEC) and Microsoft Exchange Center of Excellence.

Brief Description
Hosted Messaging and Collaboration version 4.0 Scalability Test Report

Overview
This document is to provide information around scalability testing results coming from several tests performed by Microsoft Enterprise Engineering Center (EEC) and Microsoft Exchange Center of Excellence. Tests have been performed to evaluate Microsoft® Exchange Server server roles scalability and their impact on a large Microsoft Solution for Hosted Messaging and Collaboration (HMC) 4.0 platform.