There's no solution exactly like HSTS, though DANE+DNSSEC provides a solution to the downgrade (and cert validation) problem. Postfix has an implementation. U

There's no solution exactly like HSTS, though DANE+DNSSEC provides a solution to the downgrade (and cert validation) problem. Postfix has an implementation. Unfortunately it requires DNSSEC, which has issues. Also, EFF is working on a project like HTTPS Everywhere but for SMTP encryption: https://github.com/EFForg/starttls-everywhere https://github.com/EFForg/starttls-everywhere