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:

• there are many different webmail providers and maintaining a list of them is impossible (just think of all the domain owners out in the web that possibly hosts a webmail service themself)
• I don't know if it's possible to recognize custom webmail clients just by the referrer url. Here is an example of an url of the Horde webmail client: http://webmail.domain.com/imp/message.php?index=1436. Maybe we can simply check for the "mail" and "webmail" subdomains?
• many people use mail clients like thunderbird that open mail links in the default browser, so it is impossible to track all mail referrers.

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?