Denylist
A list of senders, IPs, or domains explicitly blocked — the modern term for blacklist.
A denylist (formerly called a blacklist) is a list of senders, IPs, or domains that have been explicitly blocked or restricted by a mailbox provider, organization, or community-maintained service. Public denylists like Spamhaus, Spamcop, and Barracuda are consulted by ISPs as part of inbound filtering — being on a major public denylist often means most mail to most providers will be rejected outright.
Most senders never appear on a major public denylist. Getting on one usually requires sustained spam-like behavior: sending to spam traps, exceeding complaint thresholds, or generating very high bounce rates. Once listed, the path back is provider-specific — Spamhaus has a delisting process; others have similar processes. The fix is always structural (stop the behavior that triggered the listing) before the procedural delisting will hold.
Internal organizational denylists are different — these are companies that have blocked your domain for their own reasons (often a recipient complaint or a single problematic send). These are harder to detect and require direct outreach to the affected organization.