`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.