In case your Postfix server is on another server, it's recommended to enable TLS on Postfix submission to secure the connection between SimpleLogin email handler and Postfix.
This can be enabled by adding those lines at the end of /etc/postfix/master.cf
submission inet n - y - - smtpd
-o syslog_name=postfix/submission
-o smtpd_tls_security_level=encrypt
-o smtpd_sasl_auth_enable=yes
-o smtpd_tls_auth_only=yes
Make sure to set the POSTFIX_SUBMISSION_TLS variable to true in the SimpleLogin simplelogin.env file.