IP Blacklist: Contains IP addresses that have been identified as sources of spam or malicious activity. For instance, if a mail server at a specific IP address sends out a large volume of spam, that IP address can be blacklisted.

Domain Blacklist: Contains domain names that are associated with sending spam. If emails from a particular domain are frequently reported as spam, that domain can be blacklisted.

Email Address Blacklist: Specific email addresses known for sending spam can be blacklisted, though this is less common due to the ease with which spammers can create new email addresses.

List Maintenance: Organizations or automated systems monitor email traffic and compile lists of IPs or domains that exhibit spamming behavior.

Checking Mechanisms: When an email is sent, the recipient's ESP or email security service checks the sender's IP or domain against known blacklists. If there's a match, the email may be blocked, quarantined, or sent to the spam folder.

Reputation Impact: Being on a blacklist damages the sender's reputation, making it harder for their emails to reach recipients' inboxes.

Spamhaus: A widely-used organization that maintains several types of blacklists, including the Spamhaus Block List (SBL) for IP addresses and the Domain Block List (DBL) for domains.

Barracuda: Provides a comprehensive blacklist known as the Barracuda Reputation Block List (BRBL), which is used by many email security systems.

SORBS (Spam and Open Relay Blocking System): Maintains multiple blacklists covering IP addresses known to send spam, operate open relays, or exhibit other unwanted behaviors.

URIBL (Uniform Resource Identifier Blacklist): Focuses on domains found in the body of spam emails, rather than the sending domain or IP address.

Email Delivery Issues: Emails sent from blacklisted IPs or domains are likely to be rejected or sent to spam, leading to reduced deliverability rates.

Business Consequences: For businesses, being blacklisted can result in lost communication with customers and potential damage to brand reputation.

Efforts to Delist: Organizations on a blacklist must often go through a delisting process, which involves addressing the cause of the listing (such as fixing security issues or ceasing spam-like activities) and requesting removal from the blacklist.

Maintain Good Sending Practices: Use double opt-in methods for email subscriptions, avoid sending unsolicited emails, and keep the mailing list clean by removing inactive or invalid addresses.

Monitor Email Reputation: Regularly check your IP and domain against known blacklists and use email monitoring tools to track your reputation.

Secure Your Email Infrastructure: Implement measures like SPF, DKIM, and DMARC to authenticate your emails and prevent unauthorized use of your domain for sending spam.

Spamhaus Blacklisting: A company’s email server IP is blacklisted by Spamhaus after a security breach leads to their server being used to send spam. The company’s emails start bouncing back or landing in spam folders. They must secure their server, stop the spam, and request removal from Spamhaus’s list.

Barracuda Reputation Block List (BRBL): A marketing agency sends a campaign to a poorly-maintained email list, resulting in many recipients marking the emails as spam. The sending IP is added to Barracuda’s blacklist, affecting the agency's ability to reach clients. They need to clean up their email practices and apply for delisting.

Domain Blacklisting by URIBL: A retail website’s domain is blacklisted by URIBL after their site is hacked, and the attackers insert spammy links into their email campaigns. This causes their promotional emails to be blocked. They must secure their site, remove the malicious links, and seek delisting from URIBL.