DNSBL CheckThis tool checks if your mail server is blacklisted on one or more DNS-based blackhole lists.
For more information about DNSBLs click here.
It is also possible to use this service via a REST-API. Check out the API documentation for more infos.
Be aware that it is only allowed to use this service up to 10 times a minute. If you send too many requests, you will be blocked for some time.