One of the most impactful Exchange 2010 Client Access features are Mail tips. There's a lot more info and examples of what they can do at the MS Exchange Team blog here:
You configure Mail Tips via Powershell. These are organization level changes. You can view them using Get-OrganizationConfig and configure settings using Set-OrganizationConfig.
Unfortunately, the Exchange 2010 documentation does not at this time have documentation for each of these fields, but this is the link in case this posting dates it self.
This enables Mail Tips for reply/all including an outside the organization user
This is the service that does group metric counts for groups and nested groups
Additional things to know
- MailTips only work on Outlook 2010 (not yet released) and OWA 2010.
- Unfortunately, custom logic mail tips are not available in the upcoming Exchange 2010 release. The above options are the extent of MailTip customization