Check a sending IP against the blocklists receivers actually consult.
Blocklists are what a receiving mail server consults before accepting a connection. A listing on a widely used one means your mail is being rejected or spam-foldered right now, at scale, and usually without a bounce you would recognise as the cause.
A listing is almost always a symptom rather than a cause. The common origins are a compromised mailbox sending on your behalf, a misconfigured relay accepting mail it should not, a marketing send to an old list, or simply a neighbour on shared hosting. Delisting without fixing the origin gets you relisted, often within hours, and repeat listings are treated more harshly.
We check a deliberately short list of reputable blocklists rather than fifty obscure ones. A checker that queries every list in existence produces alarming results that mean very little, and people act on them.
Find the cause first. Check for compromised accounts and look at what your server has actually been sending. Then use the delisting page of the specific list; each runs its own.
Unknown, not clean. Several lists rate-limit queries, and we report a non-answer as unknown rather than folding it into a pass.