docker-postfix/main.cf

# SERVER
inet_interfaces = all
# $myhostname is as prefix is a RFC requirement
smtpd_banner = $myhostname ESMTP $mail_name quid agis?
# RCPT TO matches $relay_domains => !reject_unauth_destination
smtpd_relay_restrictions = reject_non_fqdn_recipient, reject_unauth_destination
# include TLS protocol & cipher in 'Received' header
smtpd_tls_received_header = yes
# + sasl username
smtpd_sasl_authenticated_header = yes

# CLIENT
smtp_tls_security_level = secure
smtp_tls_secure_cert_match = nexthop
# exceptions where secure nexthop policy is too strict
smtp_tls_policy_maps = hash:/etc/postfix/smtp-tls-policy-map
# trusted CA for exceptions specified in policy map (lvl verify & secure)
smtp_tls_CAfile = /etc/postfix/smtp-tls-trusted-ca.pem
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3
smtp_tls_session_cache_database = btree:${data_directory}/smtp-tls-session-cache
# DANE TLSA records are validated with DNSSEC
smtp_dns_support_level = dnssec
# DANE validation requires DNS lookups
smtp_host_lookup = dns

# http://www.postfix.org/COMPATIBILITY_README.html
compatibility_level = 2