The problem is that KDE comes in big lumps. You have packages like kdelibs, kdebase, kdenetwork, kdemultimedia, kdegraphics, kdepim, and so on. You can see that each package comes with several libraries and/or applications. kmail is in kdepim. But check out what else is in there before removing it!!!
The following command:
grep -H kmail /var/log/packages/kde*
will list any appearance of kmail in the installed kde packages. You will find that kmail is in the kdepim package, though there is something called kmailservice in kdelibs, also some icons in kdeartwork, translations in any kde-l10 package you might have installed and kmail-migrator in kdepim-runtime.
If you want to see what else you will be removing in a certain package you can use something like: