SMTP transport security mode. `NONE` uses plain TCP, `SSL_TLS` opens a TLS socket immediately and `STARTTLS` represents opportunistic TLS upgrade after a plain SMTP greeting. `STARTTLS` is intentionally exposed at API level even though the current runtime cannot upgrade an existing `Socket` to `SSLSocket` yet.

import klyn.mails

print(MailSecurity.SSL_TLS)