I think that webmail sites (e.g.: mail.google.com) should be shown in a separate category, and not with external websites: it's a traffic category that's different from normal websites.
I kind of like the idea of separating mail referrers from the external websites but I think this is not as easy as it seems:
If it turns out that it's impossible to track desktop mail clients and custom webmail services then tracking only five big mail provider will get you to useless information. The power of the mail network is its distributed character
If the webmail service is via https, then there may not be a referer.
Instead of a webmail category, a social media category would be more desirable. Inbound visits from twitter, digg, mail clients, etc are all indicators of your site's buzz.
Closing for now as it isn't high on the wishlist. Maybe a candidate for a third party plugin?