Deliverability
Email blacklist
A blacklist is a published list of domains or IP addresses known for sending spam. Receiving servers check incoming mail against them, and being listed can stop delivery entirely rather than just filtering it.
The major ones — Spamhaus above all — are consulted by a large share of mail servers, so a listing there is not a partial problem. Smaller lists vary in influence and in how responsibly they are run.
Delisting is a process, not a request. Fix the cause first, whether that is a compromised account, an unauthenticated sender, or a batch sent to a bought list, then submit a request explaining what changed. Asking to be delisted while the cause is still running gets you relisted within days.