This seems to have since then been fixed in upstream trunk.
As for the Kubuntu fix, pitti suggests: "we could do a manual upload of language-pack-kde-nl, put that single po file into it, and upload to -proposed, so that it'll be fixed right after install"
This was a mistake in the upstream translation at the time when it was imported http:// websvn. kde.org/ tags/KDE/ 4.3.2/kde- l10n/nl/ messages/ kdepim/ kmail.po? revision= HEAD&view= markup
This seems to have since then been fixed in upstream trunk.
As for the Kubuntu fix, pitti suggests: "we could do a manual upload of language- pack-kde- nl, put that single po file into it, and upload to -proposed, so that it'll be fixed right after install"