Microsoft published several technical solution papers on how to design, test and verify Exchange 2010 solutions for a certain number of mailboxes for a few vendors. The papers are as follows:

• Single site, 16,000 mailboxes
  Solution based on IBM X3650 M3, XIV Storage System 2810 and Brocade ServerIron ADX 1008-1 Load Balancers
• Single site, 500 mailboxes
  Virtualized solution based on on Dell T610, Windows Network Load Balancing
• Two sites, 9,000 mailboxes
  Virtualized solution based on Dell M610, Equalogic and F5 BIG-IP 3900 Load Balancers

The documents walk you through the steps regarding design decisions, challenges while meeting customer requirements.