I have received a mail with many recipient in To: as well as a "X-YMail-OSG:" header (from AOL). Both headers are long, but correctly folded. When I now bounce that mail these lines get unfolded, breaking the 1000 char length limit of SMTP and are correctly rejected by my mail server. KDE Frameworks Version: 5.45.0 Qt Version: 5.9.4 This could be related to bug 409169, I'm unsure if it hits the same code paths.