Hello.

First of all, excuse me for my bad english..

So, KMail. It will be GREAT, to be able to group some letters or threads. If You have 10 threads - no problem. If You have 50 and more active threads - it's a headache with Notes, ToDos and etc.
Second one wish.. expand threads with important, for example, messages. And do not search these marked messages threw tens of threads. Can't imagine how is to work with hundreds of active threads, but somewhere, there is a man who should do it everyday )
Third one ) Right click on thread/letter and make a folder to receive all future messages from this address into this folder. It's not comfortable to make filters, when you work with +10 threads everyday, and many of threads removes everyday.

Hope, You will can understand this text )
Anyway, Thank You for Your work!

You may want to post this request to bugs.kde.org: the forums are mainly used for discussion and user support, while bugs.kde.org is more used by developers.