The server TLS security level. Enable TLS by configuring at least may.
https://www.postfix.org/postconf.5.html#smtpd_tls_security_level.
Declarations
Type
one of "none", "may", "encrypt"Default
if config.services.postfix.settings.main.smtpd_tls_chain_files != [ ] then "may" else "none"
Example
"may"