I've only used it under Linux/BSD (there is a Windows download) and is a fast, lightweight client that has plugins for HTML viewers, image viewers, etc. The initial configuration is very basic but quite configurable.
Claws Mail is in fact a fork of Sylpheed (started life as Sylpheed-Claws), and as I understand it's basically Sylpheed with more plugins and features, so probably very good as well.
I think the latest Thunderbird can be tamed to a certain extent. You need to mess with the buttons and install some plug-inss...
.. eM might be wroth a punt but I didn't like it...
I have been using Microsoft Outlook 2000 for years. It's obviously pretty old by this point but it still works with the major mail services like gmail and it is not too hard to find for a few dollars, plus no phoning home so you won't find a copy that has been activated too many times.