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.
{
"binaries": [
{
"binary_name": "kde-config-mailtransport",
"binary_version": "17.12.3-0ubuntu4"
},
{
"binary_name": "kmailtransport-akonadi",
"binary_version": "17.12.3-0ubuntu4"
},
{
"binary_name": "libkf5mailtransport-data",
"binary_version": "17.12.3-0ubuntu4"
},
{
"binary_name": "libkf5mailtransport-dev",
"binary_version": "17.12.3-0ubuntu4"
},
{
"binary_name": "libkf5mailtransport5abi2",
"binary_version": "17.12.3-0ubuntu4"
},
{
"binary_name": "libkf5mailtransportakonadi5",
"binary_version": "17.12.3-0ubuntu4"
}
]
}
{
"binaries": [
{
"binary_name": "kde-config-mailtransport",
"binary_version": "19.12.3-0ubuntu1"
},
{
"binary_name": "kmailtransport-akonadi",
"binary_version": "19.12.3-0ubuntu1"
},
{
"binary_name": "libkf5mailtransport-data",
"binary_version": "19.12.3-0ubuntu1"
},
{
"binary_name": "libkf5mailtransport-dev",
"binary_version": "19.12.3-0ubuntu1"
},
{
"binary_name": "libkf5mailtransport5abi2",
"binary_version": "19.12.3-0ubuntu1"
},
{
"binary_name": "libkf5mailtransportakonadi5",
"binary_version": "19.12.3-0ubuntu1"
}
]
}