In KDE KMail 19.12.3 (aka 5.13.3), the SMTP STARTTLS option is not honored (and cleartext messages are sent) unless "Server requires authentication" is checked.
https://nostarttls.secvuln.info
https://bugs.kde.org/show_bug.cgi?id=423423
Source: Mitre, NVD
Published: 2021-08-10
Updated: 2021-08-20
Base Score: 3.5
Vector: CVSS2#AV:N/AC:M/Au:S/C:P/I:N/A:N
Severity: Low
Base Score: 5.3
Vector: CVSS:3.0/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
Severity: Medium