After "they" included kmail into kontact (I think it was so that it's a full PIM, like Evolution), I just ended up, leaving the email client window open and then opening/closing the app. So whenever I opened it, it always just opened in the mail client section (the only other bit I bothered with was the address book anyway, so.....).
You could also try Evolution (actually I used to have that installed as well - so it depended on what mood I was in
). Or of course, theres the option of Mozilla mail (which comes with the Mozilla browser), same for opera (which like the full mozilla is available for linux, windows etc etc) as is the seperate Thunderbird mail client and firefox browser - which can easily be configured to work together.