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Email blacklists are lists of domains, and IP addresses reported to send spam in the past.
When you send an email, the receiving mail server may check the mail headers for IP addresses and domains. If it finds any of these are listed in the blacklist provider they subscribe to (most mail servers only subscribe to one listing company for cost and resource reasons), it will be flagged. Then, they will either reject the mail silently, send a 'bounce' message that explains why it was not delivered, or deliver it to the recipient's "junk" or "spam" folder.
Most blacklist maintaining companies get their reports from third-party companies or organizations that contain sample spam messages, email addresses, IP addresses, and domains.
Some blacklist providers will automatically de-list an IP address if no new spam has been reported from it in the previous weeks or months. However, many of these providers need to be told manually by the IP address owner (Hostwinds, assuming you're hosted here) that the IP has been leased to a new client and they are not sending spam, or corrective action has been taken to prevent future spam reports.
Blacklist providers typically take 12-48 hours to update their lists with the de-listed addresses removed and depending on the mail server being sent to. Therefore, it may be long before they update their copies of the blacklist(s).
Written by Hostwinds Team / June 5, 2021