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.
The S/MIME specification allows a Cipher Block Chaining (CBC) malleability-gadget attack that can indirectly lead to plaintext exfiltration, aka EFAIL.
PHP remote file inclusion vulnerability in kmitaadmin/kmitam/htmlcode.php in Kmita Mail 3.0 and earlier, when registerglobals is enabled, allows remote attackers to execute arbitrary PHP code via a URL in the file parameter.