According to Net Applications, IE6 and IE7 combined marketshare is 2.1%. Excluding China it's even less. A lot of people might still be using Outlook 2000-2003, but they are almost certainly running at least IE8.

To me it doesn't make sense for email to support these old browsers if the websites they link to don't support it themselves.

The only problem I can think of is IE Compat View (IE 7) and Quirks Mode (IE5.5). The only mail clients I know of that trigger these modes in IE are Bigpond.com and Windows Phone (the latter of which can be fixed with a metatag).

Is it still worth supporting old IE?