outdated information on how-to/prevent-spam-in-postfix

[DocNielsen]

re https://docs.rackspace.com/support/how-to/prevent-spam-in-postfix/

abuseat.org is now part of spamhaus, so can be removed.
barracudacentral.org requires registration.

You need to look at return codes, when you query the RBLs. Otherwise errors would also cause email to bounce.
The correct lines would be

reject_rbl_client zen.spamhaus.org=127.0.0.[2..11],
reject_rbl_client bl.spamcop.net=127.0.0.2,
reject_rbl_client all.spamrats.com=127.0.0.[36..38],
reject_rbl_client dnsbl.sorbs.net=127.0.0.[2..15],