Edit /etc/postfix/master.cf file

Original Value is like that:

submission inet n – – – – smtpd -o smtpd_enforce_tls=yes -o smtpd_tls_security_level=encyrpt-o smtpd_sasl_auth_enable=yes -o smtpd_client_restrictions=permit_sasl_authenticate d,reject -o smtpd_sender_restrictions= -o smtpd_proxy_filter=127.$

Change it as like as this one

submission inet n – – – – smtpd -o smtpd_enforce_tls=no -o smtpd_tls_security_level=may -o smtpd_sasl_auth_enable=yes -o smtpd_client_restrictions=permit_sasl_authenticate d,reject -o smtpd_sender_restrictions= -o smtpd_proxy_filter=127.$

Then start postfix, that removes STARTTLS requirement at postfix submission service.

By the way, the command to make postfix accept the changed settings is:

postfix reload