It looks like my understadning of RFC3676 was incorrect -- the requirement to wrap lines probably applies to the body part text *prior to any Content-Transfer-Encoding transformations*. Trojita currently produces wrapped lines, but only thanks to the quoted-printable CTE. Thanks to Michał Górny <mgorny@gentoo.org> on gentoo-core for reporting this.
Git commit 2111ef9319396ed72de52452796e989625cc9ab5 by Jan Kundrát. Committed on 03/08/2013 at 10:15. Pushed by jkt into branch 'master'. Merge branch 'format-flowed' REVIEW: 111842 http://commits.kde.org/trojita/2111ef9319396ed72de52452796e989625cc9ab5