To disable TLS in the master.cf file for postfix - Change this line in the /etc/postfix/master.cf:

submission inet n - n - - smtpd -o smtpd_enforce_tls=yes -o smtpd_tls_security_level=encrypt -o smtpd_sasl_auth_enable=yes -o smtpd_client_restrictions=rmit_sasl_authenticated,reject -o smtpd_sender_restrictions= -o smtpd_recipient_restrictions=permit_mynetworks,permit_sasl_authenticated,reject_unah_destination

to the following:

submission inet n - n - - smtpd -o smtpd_enforce_tls=no -o smtpd_tls_security_level=may -o smtpd_sasl_auth_enable=yes -o smtpd_client_restrictions=rmit_sasl_authenticated,reject -o smtpd_sender_restrictions= -o smtpd_recipient_restrictions=permit_mynetworks,permit_sasl_authenticated,reject_unah_destination

and then restart postfix:
service postfix stop
service postfix start