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using all email addresses in Kaddressbook as a whitelist for filters.
either through adding the addresses to bogofilter/spamassassin itself or by creating a filter (with anti-spam wizard) that checks if new mail is found in addressbook before piping it through the spamfilter.
whichever works better..

adding all addresses you send emails to to this whitelist would be a nice addition.

this would greatly reduce the number of false positives

found a project on sourceforge that did something like that for kde3. https://sourceforge.net/projects/kab2wl/
seems to be dead though
maybe the source code could help the developers implement this feature