We support MTA-STS (RFC8461): we publish a generated policy and hooked a resolver to Postfix.
Our zones are DNSSEC secured and we publish DANE-EE TLSA DNS records for the leaf certificates. Postfix is configured to validate if the records are available.
SPF / DKIM / DMARC¶These are flawed:
The world did not deploy SRS massively, mailing-lists are still common and often altering the mail, we do support SMTP SUBMISSION for roaming users but many providers or companies still block legitimate ports, mail forwarding and redirections on another domain are also useful features, so we decided to not implement these.
Hardenize does extensive checks for mail; it is a bit excessive about accepted ciphers: it's still better to do opportunistic TLS with a medium strengh cipher than go encrypted.