Why does my test show a list as unreachable?

Spamhaus and others answer queries from public resolvers such as 8.8.8.8 with error codes instead of a result. Operators should use their own resolver.

Is checking blacklists enough?

No, they are one part. Reverse DNS, authentication and content belong to deliverability as well.

Blacklist check: is my mail server IP listed? Causes and delisting

Blacklists (DNSBL) collect IP addresses that sent spam or malware. If your sending IP sits on a common list, many servers reject your mail or move it to the spam folder.

How the lookup works

A DNSBL is a DNS zone. The receiver reverses the IP, appends the name of the list and asks the DNS server. An answer in the range 127.0.0.x means: listed.

203.0.113.25  ->  25.113.0.203.zen.spamhaus.org

The main lists

Spamhaus ZEN combines SBL, XBL and PBL. SBL and XBL stand for spam sources and compromised machines, PBL for address ranges that should not run mail servers.
SpamCop (bl.spamcop.net) lists based on user complaints.
Barracuda (b.barracudacentral.org) and further lists feed into many filters.

Why an IP gets listed

A compromised mailbox or hacked server sends spam.
An open relay or an insecure contact form is abused.
The IP belongs to a consumer (dynamic) range or has a bad neighbour on a shared server.

Delisting

Find and fix the cause: lock accounts, change passwords, close the holes.
Then file a removal request on the list's website. Some lists remove entries on their own after a waiting period.
For PBL entries send through the provider's relay or request a release from the provider.

Blacklist check for your mail server IP