kontact/kmail (KDE mail ua) used to integrate all that stuff the only thing it lacked was an ability to handle HTML email. At the time the developers kept insisting that email should all be plaintext and they weren't going to allow HTML mailing in kmail.

I moved to thunderbird.

The rest of KDE4 caught up to being release standard at about version 4.6 and Kontact looks good but I've become entrenched with TB now.