To install on Debian/Ubuntu:
apt-get install freepops
sudo freepops-updater-dialog
To download sent mail folder set the username as username@webmail.com?folder=Sent.
If not specified "Inbox" is assumed.
With kmail, one can create seperate directories (ex. yahoo [inbox], yahoo [sent]) and associate accounts with these folders. This way messages remain more organized and they can simply be dragged and dropped to the gmail's "all mail" directory later on, to save all the mails on google's massive servers with ever increasing inbox space forever, for a greater webmail experience and excellent searching capability.
Good Luck, Happy webmailing !!