But why bother?
When you think Outlook is most people's primary application (spends the most time running under my Citrix farm), does document management that integrates with SharePoint (which has a free version), forms, support for VBA, excellent calendar support including scheduling with others based on their calendars, it's lightning fast, can be managed by Group Policy, integrates with Office and has thousands of 3rd party products that integrate with it. (Email archiving software, management tools, Antivirus, even CRM solutions)
It would take companies years to recover from the lost productivity during the transition to Thunderbird or some other generic POP3/IMAP application (FOSS or not). I don't have time to dick around with "plug-ins", retraining staff, the loss of functionality compared to Outlook with Exchange, shite calendaring support, lack to syncronising with PDA's/phones etc.
I'd pay £40 a user to have that fixed thanks.
Love it or hate it, Office sorta just works. No fiddling, no checking compatibility in other apps (as Office is the defacto), rich history of VBA apps behind it, saving as PDF, SharePoint integration etc, Windows integration.
The 2007 release (and 2010 beta) is rock solid, with great management, document recovery, integration and user accecptance. The only end-user issues are normally with the more eldery IT folks (in particular devs) who refuse to use it.
For the price - the basic office apps are a bargin, with no big name single suite out there (free or not) comparing on management, intergration, features and 3rd party support.
Simple as that.