DNSBL Blacklists are a very important part of the internet these days, they identify spammy IP addresses using various methods, some DNSBLs rely on user feedback to report spam (when they mark emails as spam). but others use smarter techniques such as email content detection, DNS record validation and some monitor the density of the emails your server sends over time and if there is a sudden spike in emails being sent they will temporarily but your IP on a blacklist.
This is all great until you are blocked yourself and your customers do not get your emails any more, this can be really frustrating! Especially if you send very critical emails such as account specific emails and invoices to your clients.
Blacklist Scan will check all the popular blacklist databases from around the world and provide you with a list of blacklists you are currently in.