A blacklist, also increasingly referred to as a blocklist, is a list that includes IP addresses, domains, URLs or email servers associated with spam, malware, phishing, harmful links or suspicious sending behaviour. These lists can be used by email security systems, internet service providers, corporate networks and spam filters to identify risky sources. The purpose is to protect users from unwanted, harmful or low-trust messages.
Being blacklisted can create a serious deliverability problem for email marketing and corporate communication. If an IP address or domain appears on a blacklist, emails sent from that source may struggle to reach the recipient’s inbox. Some messages may land in the spam folder, some may be rejected completely or some may be delayed by the receiving server. However, an email going to the spam folder does not always mean that the sender is blacklisted; sender reputation, content quality, recipient engagement and technical configuration also affect deliverability.
An IP address or domain can be blacklisted for many reasons. Bulk and unsolicited email sending, high spam complaint rates, excessive bounce rates, use of purchased or low-quality email lists, malware distribution, phishing content or suspicious sending activity from a compromised email account are among the most common causes. Open relay configurations, incorrect DNS records, weak server security and faulty email authentication settings can also create risk.
Correct configuration of technical authentication records is important in email sending infrastructure. SPF, DKIM and DMARC records help receiving servers evaluate whether the sender is authorised to send email on behalf of the domain. In addition, reverse DNS, or rDNS, sender IP reputation, domain age, sending frequency and user engagement may also be considered. Missing or incorrect configuration can reduce email trust and increase blacklist risk.
One common example of blacklisting risk is a high complaint rate. For example, if a company frequently sends promotional emails to users who did not give permission or are not interested, many recipients may mark those messages as spam. Over time, this can reduce the reputation of the sender domain or IP address. Similarly, sending large volumes of email to old lists that have not engaged for a long time can also be risky. For this reason, email lists should be permission-based, up to date and properly segmented.
Sending a test email to another address is not enough to determine whether you are blacklisted. Whether a test email reaches the inbox or lands in spam does not provide a definitive result on its own. A more reliable approach is to use blacklist checking tools such as MXToolbox, Spamhaus, Barracuda, SURBL or SpamCop. In addition, deliverability reports from the email service provider, bounce messages and error codes returned by receiving servers should be reviewed regularly.
To be removed from a blacklist, the root cause should first be identified. If an account has been compromised, security measures should be taken; malware should be removed, open relay should be closed, DNS and authentication records should be corrected and the application or user causing spam sending should be identified. Requesting delisting before fixing the underlying issue usually does not provide a permanent solution. Some blacklists may remove listings automatically after a period of clean behaviour, while others require a manual removal request.
Changing the IP address is not always the right solution. If the underlying problem continues, the new IP address may also gain a poor reputation quickly. A better approach is to fix the sending infrastructure, maintain list hygiene, use permission-based marketing, reduce user complaints and manage sending volume carefully. For brands that run email marketing, regular bounce cleaning, unsubscribe mechanisms, segmentation and frequency control are especially critical.
In summary, a blacklist is an important security mechanism used by email and internet security systems to mark sources considered risky. Being blacklisted can negatively affect email deliverability, brand credibility and marketing performance. To reduce this risk, technical email settings should be configured correctly, permission-based databases should be used, spam complaints should be monitored, sender reputation should be protected and security checks should be carried out regularly.
