Kmail2 has been a nightmare for me as well. I've switched to thunderbird and while it's not terrible, I don't particularly care for it. I'm hoping kmail2 will eventually be un-fucked so I can switch back.
In KDE 4.7, I have issues where sometimes when I click on apps in the task manager, it doesn't bring them up. I get this on my desktop and laptop (Kubuntu). It generally takes another click or 2 for it to work.
I have another issue where sometimes I get 2 programs overlapping in the task manager (you see the text of both), but only one of them is accessible. I can fix it by right clicking, and moving the accessible app to another virtual desktop, then moving it back. Then both apps have distinct task bar entries that work. I had this problem in 4.6 as well, but it also happened much more frequently in 4. so I guess there's been some improvement.