It's been quite a while since I used it, but I recall Kmail being quite customizable. Indeed I actually remember it as being more customizable and had kind of been meaning to switch back to it from Thunderbird (but haven't actually done so because switching mail clients is a pain).
kmail is in terrible state and has been for while (can't even send text email by default for several weeks (months?). IINM a replacement is being developed and kmail is on life support until then.
