SUMMARY *** NOTE: If you are reporting a crash, please try to attach a backtrace with debug symbols. See https://community.kde.org/Guidelines_and_HOWTOs/Debugging/How_to_create_useful_crash_reports *** STEPS TO REPRODUCE 1. Compose and send email using KMail 2. 3. OBSERVED RESULT Error message: "Failed to transport message. Error during SSL handshake: error:1408F10B:SSL routines:ssl3_get_record:wrong version number". Email remains in Outbox. EXPECTED RESULT Send email as usual without any problems. SOFTWARE/OS VERSIONS Windows: macOS: Linux/KDE Plasma: Slackware 64/ KDE Plasma 5.23.4 KDE Plasma Version: 5.23.4 KDE Frameworks Version: 5.59.0 Qt Version: 5.15.3 ADDITIONAL INFORMATION Using Sympatico email via IMAP. Sending server is smtphm.sympatico.ca. Port 587 with STARTTLS. All other email apps (Seamonkey Mail, Thunderbird, Pegasus, Windows Mail) work as usual. Problem started in KMail 21.12.0.
Closing the ticket. The problem seams to be in my Postfix relay configuration (which hasn't changed). Looks like my ISP changed something.