A while ago, we began the process of documenting the scalability limits of
both Exchange 2007 SP2 and Exchange 2010. We've looked at all areas: things like
organization sizing, rules sizing, IRM-protected messages, secure channels,
named properties, load balancing, network latency, and more.
There's a lot of great information in this spreadsheet, including issue descriptions, mitigations,
and links where appropriate.
For more information on this download see:http://blogs.technet.com/b/exchange/archive/2010/03/12/3409421.aspx