oh how i love linux… it just makes sense 
i just got a new laptop and was in the process of moving my stuff to the new one (i guess i could’ve just copied over my whole $HOME but i wanted to take the opportunity to sort out some old stuff). before migrating my kmail-contents, i of course googled it first.
there were suggestions a la “create a new folder in the mbox-format and copy all your mails in there, copy the folder to the new computer and import in into kmail”… what the hell? i have hundreds of emails neatly organized in a nice folder-structure – i’m certainly not gonna just throw it all into a single folder. moreover this sounded like real work, which needs to be avoided
so here comes my attempt, which yielded instant success:
in a terminal, go to:
cd ~/.kde/share/apps
and tar the kmail-folder your’ll find there up with:
tar -cf kmail.tar kmail
just copy this kmail.tar to the new computer and extract it into ~/.kde/share/apps.
attention: you need to tar it in order to preserve file attributes, if there are symlinks or alike inside this folder, an ordinary copy like using dolphin over sftp:// will probably not work correctly!
then you need to copy oldmachine:/.kde/share/config/kmail* to the coresponding folder on the new machine. fire up kmail and be impressed how this easy process “just works” 
you’ll find your old beloved folder-structure and account-settings (i recommend re-configuring the passwords as i assume kwallet won’t find them, of course), as well as filters just working out of the box. also font-sizes and your favourite folders will be applied correctly.
in my case i also had a filter for handling spam with crm114 (thoroughly trained with ~10 000 spam and ham mails imported from thunderbird 6 months ago), so i also copied (again, first tar it up) ~/.crm114 to my new machine, which still needs to show if it works correctly, no spam arrived in the last 24h
actually the fact that this works will be no surprise to you if you’ve been using linux for some time, because it is kind of the same thing that happens if you reinstall some application or make a distro-upgrade that preserves $HOME – all app-specific settings are in $HOME and will be understood by the program. i hope this helps someone, i didn’t find any suggestion like this with google, but maybe my google-skills are just not good enough…
oh it might make sense to mention that my kde-versions on both machines are (almost) the same, kde 4.5rc1 on the old one, kde 4.4 on the new one.
