Version:            (using KDE 3.5.9)
Installed from:    SuSE RPMs

Setting and clearing CC/To/BCC fields are common task -- surprisingly setting comboboxes are placed most-left, and clear button are places most-right. In effect every user has to move mouse from one edge of the screen to the other, I hope it is obvious that those widgets are the same "kind" so they should be placed together, not scattered around composer window.

I would opt for such order:
[x> [ type ] [ field edit box ]
but it is only a suggestion.