The email client market share as of January 2010. These numbers are not exclusive – some people used more than one email client during the month, and so that will register a vote for each client used.
But beware: GMail and Outlook2007 are underrated in this statistic: The email client a person is using can only be detected if images are displayed. This can give an inflated weighting to email clients that display images by default, such as Outlook 2000 and the iPhone. It will also provide a lesser weighting to those that block images by default such as Gmail and Outlook 2007.