Postfix is no longer using the RBL lists that I have configured it to use.

First check that the positioning of the rbl call in your postfix main.cf file is correct under smtp clients.

Here is an example for Postfix v 1.

maps_rbl_domains = bl.spamcop.net

smtpd_recipient_restrictions =

reject_invalid_hostname,

reject_non_fqdn_sender,

reject_non_fqdn_recipient,

reject_unknown_sender_domain,

reject_unknown_recipient_domain,

reject_unauth_pipelining,

permit_mynetworks,

reject_unauth_destination,

reject_maps_rbl,

permit

Then tail your mail logs and check to see if the mail server is attempting to query the RBL in question.

If there is no trace in the mail log of a query attempt of the RBL cannot be contacted then manual try to query the rbl domain from the command line.

E.g. ~$ host bl.spamcop.net

If the query does not return a result then your dns server cannot resolve the ip for the rbl domain.
Go to /etc/resolv.conf and check to see if the correct external, public dns server(s) are listed in the correct order.

Try again once you have resolv.conf configured correctly and you can do nslookups then the RBL should be queried correctly. Restart Postfix and tail the maillog again to check this.