I just figured out how to automatically add addresses I wrote an email to to my addressbook in kmail. Christian Schaarschmidt wrote a small patch which adds a filter you can define for your outgoing mails. Unfortunately it’s not available in vanilla kmail (yet?). I added an updated ebuild+patch to my gentoo repository so one can easily use it.

I think got used to that feature with the previous mail client I used: thunderbird and I am wondering if the mail client I used before thunderbird (mutt) has a similar feature.