KMime is missing some functions to format those headers in a mailserver save way (max linelength 76chars). This ends up in several places where people developed the same thing again and again:
https://invent.kde.org/pim/kmail/-/blob/master/src/editor/kmcomposerwin.cpp#L1655
https://invent.kde.org/pim/kmail/-/blob/master/src/editor/kmcomposerwin.cpp#L1674
https://invent.kde.org/pim/messagelib/-/blob/master/messagecomposer/src/job/autocryptheadersjob.cpp#L127