Matthew Allen, an old friend of mine, who is an incredibly talented programmer who dreams in windows C++ api calls wrote a email client a few years ago. Personally I have never used it, but I do know, given that we worked together for a while and even were in a band together, that Matt knows his stuff.
He was complaining about the lack of google results for ’email client’ these days. So Matt here is some link love. Hope it helps or alternatively humans reading this blog could check out Scribe.