I've got another solution for you, and it's your old trusted SendMail.exe to be used in conjunction with Stunnel. Stunnel does support STARTTLS, and it runs under Linux/Windows/MacOS/*NIX.
What's Stunnel now about, you might ask? Well, this:
Stunnel is a proxy designed to add TLS encryption functionality to
existing clients and servers without any changes ...
I would replace this with a more native approach using PowerShell. As Send-MailMessage is now obsolete, that leaves us with System.Net.Mail: MailMessage & SmtpClient. Supposedly your scripts can pass the variables (and you can fill the missing) to:
$mailMessage = New-Object System.Net.Mail.MailMessage($fromAddress, $toAddress, $subject, $body)