DNS domains for blacklist lookups
smtpd_client_restrictions: reject_rbl_client
This parameter specifies an optional list of DNS domains that publish the
network addresses of blacklisted hosts.
By default, RBL blacklist lookups are disabled.
Specify a list of RBL domains, separated by commas.
The real-time blackhole list works as follows: reverse the client
network address, and reject service if it is listed below any of
the following domains.