Re: Unable to receive mail from certain hosts

[postfix]

smtpd_enforce_tls = yes

Oops, i missed this one too:
http://www.postfix.org/postconf.5.html#smtpd_enforce_tls

smtpd_enforce_tls = yes
Mandatory TLS: announce STARTTLS support to remote SMTP clients, and 
require that clients use TLS encryption.