BTW, here is my config of the MSA in Postfix (master.cf) :
# ==========================================================================
# service type private unpriv chroot wakeup maxproc command + args
# (yes) (yes) (no) (never) (100)
# ==========================================================================
submission inet n - y - - smtpd
-o syslog_name=postfix/submission
-o smtpd_tls_security_level=none
-o smtpd_reject_unlisted_recipient=no
-o smtpd_recipient_restrictions=
-o smtpd_sasl_auth_enable=no
-o smtpd_client_restrictions=permit_mynetworks,reject
When I received Postfix from the package library of the distro I am using, Postfix as packaged had the submission logical line commented out. Maybe if I had compiled Postfix from its source as published in its repo (instead of installing the package), the submission line wouldn't have been commented out. But no doubt there were benefits to using the package. I suppose it included some aspects of default configuration appropriate to the rest of the distro, e. g. the method of setting up the auto restart of Postfix when the system is rebooted. Anyway, it all seems to be working for me now.