Query 20+ public DNS-based blocklists to see whether your sending IP or domain is listed. Mail receivers use these lists to filter spam and phishing; being on one is a leading cause of "my email keeps going to spam." We query all RBLs in parallel so the check finishes in seconds.
A Realtime Blackhole List (also called a DNSBL or DNS blocklist) is a published list of IPs or domains believed to send spam, phishing, or other unwanted mail. Mail servers query these lists during inbound mail delivery and can reject or quarantine matched messages.
Being listed doesn't mean you're a bad actor. Common causes: a shared mail server got compromised; a forwarder you used got abused; a former owner of your IP address sent spam. The fastest path to delisting is usually direct contact with the operator of each list (links in their respective sites).