Check a domain or sending IP against the major DNS blocklists, live, and see which listings actually carry weight.
A listing is not an event you can check once. Being clear today says nothing about Thursday, and the damage is done in the days before anybody notices.
A DNS blocklist of IPs or domains known for spam. Receiving servers check them and may reject or spam-folder listed senders.
Fix the underlying cause first, then use the delisting request form on the specific list. Delisting without fixing the cause usually re-lists you.
Not always. Impact depends on the list's reputation and which receivers use it. Major lists have broad effect.