I've been noticing that I get a bunch of spam from IP addresses that have no valid PTR record associated with them. Legitimate mail servers are more or less required to have this basic level of DNS functionality configured if they wish to have a shot at delivering traffic to a number of SMTP destinations on the internet. As such, I was attempting to add the "reject_unknown_reverse_client_hostname" restriction to the smtpd_recipient_restrictions line that the bundled Postfix 2.10 install picks up from the main.cf (see Postfix Configuration Parameters for more info) to knock out some of thos low hanging fruit that is clogging up my user "Junk" folders.
I've tried a number of things and can't successfully get this "reject_unknown_reverse_client_hostname" restriction picked up by the smtpd_recipient_restrictions conf entry within Zimbra's bundled Postfix instance:
1) I tried adding this to the zimbraMtaRestrictions list via "zmprov mcf +zimbraMtaRestriction reject_unknown_reverse_client_hostname". I can tell this at least works to provision the setting internally because I get the following response:
$ zmprov gacf | grep reject_unknown_reverse_client_hostname
However, "$ postconf | grep reject_unknown_reverse_client_hostname" comes up blank.
2) I tried adding it manually to the the string in the /opt/zimbra/postfix/conf/main.cf and restarting (this seems to get overwritten)
3) I tried updating Postfix directly using "postconf -e 'smtpd_recipient_restrictions = reject_non_fqdn_recipient, permit_mynetworks, reject_unlisted_recipient, reject_non_fqdn_sender, reject_unknown_sender_domain, reject_unknown_reverse_client_hostname, reject_rbl_client zen.spamhaus.org, permit'", but this too gets killed after a restart.