In this article the author summarises his experiences with various Windows email clients, and discusses their shortfalls (in his opinion). Some of the packages he's chosen are kind of obscure, but there's some known names in there, such as Eudora, Thunderbird, and Pegasus to name a few.
The client he ended up with? Outlook, because apparently it 'sucks less'. It's interesting to observe that a Notes client would address most of the shortcomings that he found in other clients, but unfortunately it might have required some developer knowledge in some areas. Everyone who sees some of the customisations I've made to the Notes client / Notes address book / Other custom databases I use daily seems to immediately see the value in them, however the problem is that I know the only reason I was able to create these things is because I'm a developer by trade.