<p>This is not "exactly" a programming question, but it's highly related.  We are writing an app that sends out email invitations for a client (no, it's not spam).  Their designer gave us an HTML and CSS template to use which is fine.  The problem is that it looks like crap in Outlook 2007 because Microsoft decided to use Word (of all things!) as the <a href="http://msdn.microsoft.com/en-us/library/aa338201.aspx" rel="nofollow noreferrer">rendering engine for HTML in Outlook 2007</a>.  I want the client to understand that they should design a "compatible" look and would love to be able to show some kind of statistics about what email clients are being used out there, namely that Outlook 2007 is growing in use.  </p>

<p>Has anyone run across any white papers, web sites, studies that even come close to providing a view on this?  I don't expect census level accuracy, but something fairly credible would be good.  Thanks for any help.</p>

you should look at
ReturnPath
- they somewhat specialize in that.
Clients you likely need to consider (aside from Outlook):
AOL
Gmail
(Google)
Yahoo Mail
(Yahoo)
Hotmail
/
Live
/
MSN
/
Outlook
(Microsoft)
Lotus Notes (IBM)
Thunderbird
(Mozilla)