You don't need to be downvoted, but my experience so far completely contradicts what you're saying (with around 50 active clients at the moment, including some businesses, and currently accruing one new client a week for the last 6 weeks).

Outlook is pretty much the de facto mail client amongst my clients, even those that aren't in business environments. My largest business client is email-program-agnostic, and still the vast majority of users are asking for Outlook.

Among webmail users, most usage is either Comcast's site, Yahoo's (a result of the SBC partnership), or their internet service provider's.

Actually, I think this might help put it in perspective: more of my clients have email addresses at their own domain versus having Gmail accounts.