Market share indicators will not help you. When developing HTML email, the only thing that matters is what the client uses, which your target group uses. It depends on the geographical area, industry, B2B / B2C - the variations in practice are huge. In some industries (journalism ...) you will even have to reckon with a huge population using clients such as Lotus Notes, which is known for supporting HTML a bit more nominally (shudders).
Outlook 2007, of course, cannot be neglected, in particular if you send business addresses, but with Vista on new PCs it also received a noticeable presence for private accounts.
The return trip does have data according to the industry.
However, in practice, a good approach is to follow the “save” recommendations in the lowest common denominator. Outlook 2007 is not the only problem client - Gmail is also quite famous for not having support for several design elements that others display just fine. You will find that an amazing number of web designers do support a sideline with HTML email design (there is demand, and this helps pay rent). If you are just starting out, Campaign Monitor (an email marketing service provider) has a wealth of good resources. You can start with the 2008 Email Development Guide . They are also one of those behind the Draft Email Standards .
Oh, I personally use Thunderbird with IMAP, Gmail, and RoundCube.
(Disclaimer / full disclosure: I really work for a competitor, in the free sense, Campaing Monitor.)
chryss
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