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
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