You might see these repeating messages on a newly perfectly installed Virtualmin server:
Jun 1 06:44:36 host postfix/smtp[104754]: warning: reason: dnssec_probe 'ns:.' received a response that is not DNSSEC validated Jun 1 06:44:36 host postfix/smtp[104756]: warning: DNSSEC validation may be unavailable Jun 1 06:44:36 host postfix/smtp[104756]: warning: reason: dnssec_probe 'ns:.' received a response that is not DNSSEC validated Jun 1 06:44:36 host postfix/smtp[104758]: warning: DNSSEC validation may be unavailable Jun 1 06:44:36 host postfix/smtp[104758]: warning: reason: dnssec_probe 'ns:.' received a response that is not DNSSEC validated Jun 1 06:44:36 host postfix/smtp[104766]: warning: DNSSEC validation may be unavailable
By come accounts you can ignore these warnings which were introduced in a Postfix 3.5.9 update:
https://forums.zimbra.org/viewtopic.php?t=70420 refers to https://www.postfix.org/announcements/postfix-3.5.9.html
You can determine you Postfix version so:
# postconf mail_version mail_version = 3.6.4
But can you? Should you? How can one turn off this clutter? Let us know in the comments what you think.