Theses are sites that instead of maintaining a blocklist they track the reputation of mail servers giving them a rating like poor, neutral, or good. If your mail server has a poor rating it means they have seen spam from you server and it’s likely anyone using their service will block your mail.
These services I’m listing are all legit and once the problem on your mail server is resolved the rating will usually go to neutral within a day.
List of reputations services and links to their lookup page
WatchGuard
Lookup URL: http://www.reputationauthority.org/lookup.php
Notes: Nice lookup tool to lookup by your domain or mail server IP. They even have a De-list request which is nice of them.
mailspike
Lookup URL: http://mailspike.org/iplookup.html
Notes: Only one I know of that you could actually use to block spam yourself at no cost. I haven’t used them myself for blocking spam.
TALOS
Lookup URL: https://www.talosintelligence.com/reputation_center/lookup
Notes: Part of Cisco, used to be senderbase.org. Looks like they are doing more than just email reputation. Never had any problems with them.