Juergen: So first, we won't require 5MB per user, but for some profiles we'll recommend it. We're in the process of finalizing our system requirements and recommendations. The amount of memory per user we recommend varies with profile. As an admin/architect, the ability (but not requirement) to add RAM will give you a powerful lever in pricing and building your Exchange Server.
A few main drivers are the amount of mailflow (send and receive), the concurrency (how many users are accessing the system at the same time), and applications used. There are MANY factors that also contribute, but usually to a lesser degree like mailbox size and folder size.
Mailflow is the biggest, to no suprise. Mailbox size drives IOPs when there are applications that perform lots of searches or frequently download the contents of the mailbox. Using Cached Mode reduces a large burden on the server of searching and sorting lots of items. Folder size is important for the previous reason and to make sure Cached Mode experience remains positive.
Microsoft
