I want to set up a small mail server. I have got Postfix on Ubuntu working, but now I want to add some level of spam filtering using the zen.spamhaus.org DNS blacklist, and this is not working. The Spamhaus instructions suggest trying this command:

dig +short 2.0.0.127.zen.spamhaus.org

and if I run it on my droplet I get no answer. At home, using my ISP’s DNS server, I do get a reply, so Spamhaus is working fine, it’s the droplet’s resolver that’s not returning anything. The /etc/resolv.conf file includes Google’s public name servers:

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 2001:4860:4860::8844
nameserver 2001:4860:4860::8888
nameserver 8.8.8.8

The Spamhaus instructions mention:

Check what DNS resolvers you are using: If you are using a free “open DNS resolver” service such as the Google Public DNS […] in most cases you will receive a “not listed” (NXDOMAIN) reply […] We recommend using your own DNS servers when doing DNSBL queries to Spamhaus.

Can I configure my droplet to use some DNS server that does return replies from zen.spamhaus.org?