In short: A blocklist entry means your domain or mail server was reported for spam or malware. Find and fix the cause first, then ask each list to remove you.
Updated
Why it happens
The most common cause is a hacked site or mail account that is sending spam or hosting a phishing page. Others are a shared mail server with a bad neighbour, a leaked password, or a domain that was abused before you registered it.
How to check
Use the free Kiplock blocklist check, which looks up your domain and its mail servers on common lists. A clean result is a good sign, but Gmail and Outlook also keep private filters.
How to fix it
1. Find and remove the cause first: scan the site for unknown files and users, change all passwords, update the software, and review who is sending mail as you.
2. Only then ask each list to remove you. Each list has its own request form. Asking before fixing gets you listed again.
3. Make sure SPF, DKIM and DMARC are set so others cannot fake your domain.
Free, no sign-up.
Run the blocklist checkCommon questions
How long does removal take?
Some lists remove you within hours after you ask, others take days. Some drop entries on their own after a clean period.
Is a blocklist the same as Google Safe Browsing?
They are different services. This check covers common spam and malware lists, not Google's own.