What is an email blacklist?
An email blacklist, or DNSBL (DNS-based blocklist), is a list of IP addresses or domains that its operator has seen sending spam or behaving like a spam source. Receivers query it over DNS as mail arrives, and a listing can mean a rejection, the spam folder or a higher spam score, depending on the receiver.
Reading the result
For an IP the tool queries 25 lists in one pass; for a domain, only the lists that index domains apply (1 today), so check your sending IPs too. Each list shows as listed, clean or unchecked, and one that does not answer is unchecked, never clean. Spamhaus, CBL and URIBL are outside the count: they refuse queries made through public DNS resolvers.
Beat runs on the same engine, on a schedule, across all 41 checks and 25 blacklists. What it adds is asking again, noticing when an answer changes and telling you what to do about it.
Common blacklist problems
Checking the domain when the IP is listed
IP lists such as SpamCop and Barracuda judge the sending server, while domain lists such as SURBL and URIBL judge the domains in your links. Check the domain and the IP.
A listing on a shared IP
On a shared sending pool, someone else's traffic can list the IP you use. Ask your provider to request removal or move you to a cleaner pool.
A domain on SURBL or URIBL
Every message that links to the domain is at risk, whichever server sends it. A shared link-tracking domain can put you there without anything you sent.
A range-wide listing
UCEPROTECT levels 2 and 3 list whole network ranges, so you can be listed because of your neighbours.