This migrates the security level for outgoing SMTP connections to
dane[1]. Either a server is configured for DANE or it now uses mandatory
unauthenticated TLS.
If DANE validation fails, the delivery will be tempfailed.
If DANE is invalid or unusable the connection will fall back to
unauthenticated mandatory TLS
This has been the default in various mail distributions:
- Mailcow since December 2016[2]
- mailinabox since July 2014[3]
[1] https://www.postfix.org/TLS_README.html#client_tls_dane
[2] 47a5166383
[3] e713af5f5a
fac7efe946