A program called Fingerprint from Litmus is generating statistics on e-mail client market share. I was surprised to see Gmail's share so low (6% amongst business users, 4% amongst consumers) but also how old most Outlook client installs are (7% business users using Outlook 2007 versus 29% using Outlook 2003 or earlier).
At least Lotus Notes is down to 0.2% of business users.
The authors suggest running their software on your own mailing list. Presumably their analysis is a bit more sophisticated than what you'd get by looking at the Browsers dimension in Google Analytics for a specific "e-mail only" image.
More discussion on the Litmus Blog.