Version: 1.8.2 (using KDE 3.4.2, compiled sources) Compiler: gcc version 3.4.2 [FreeBSD] 20040728 OS: FreeBSD (i386) release 5.4-STABLE When setting a custom folder icon, the icon set in the dialog box does not default to system icons: filesystem, which is the logical place to find the needed icons. Instead it defaults to "other icons" which appears to be the default icon set regardless of whether or not a custom icon set is selected. This means that if the user wants to select custom icons that will track changes in the icon theme they have to click the "system icons" radiobutton, wait while all of those load, select "filesystem" from the dropdown, wait while all of those load and select the icon they want. I configure kmail folders to be folder_grey when they have no mail in them and folder_red when there is new/unread mail in them. If I want those icons to follow changes in my icon theme then I have to go through the process of switching icon theme in the properties dialog and waiting for the system icons to load twice for every folder, which takes an incredible amount of time. Usually, I just end up editing kmailrc with sed instead, because it's easier :)
I think the point here is that if the icons are chosen from the 'other icons' section, in kmailrc the icon is stored as the full path to the icon. Whereas if the icons are chosen from the 'system icons' sections, just the icon name is stored, which means if the icon theme is changed it'll use that icon from a different theme. The system icons-filesystem section seems to contain the same icons anyway.
in KMail2, the default is "Places" which is a more natural choice than system icons.