Within your email marketing stats, you may occasionally see a bounce that has been recorded for an email you sent, and the reason for the bounce refers to a blacklist. Bounces are a normal part of sending email and so are blacklists, but that doesn’t mean they shouldn’t be a cause for concern.
So, what is a blacklist? To prevent spam from clogging people’s inboxes, email service providers use a variety of tools to determine if an email is legitimate and desired by the intended recipient. One such tool is an RBL, or remote blacklist. An RBL is a list of known or suspected IP addresses that are considered to be sources of spam. There are hundreds, if not thousands, of RBLs in the world. Some of them are maintained by large companies, some are open source projects run by volunteers and some are run by a single (very devoted) person. A blacklist’s legitimacy and impact can vary as much as the size.
Blacklists are compiled in a few different ways. Some blacklists use manual reporting mechanisms where participants report suspected IP addresses to the blacklist. If enough people report the IP address, the blacklist may add it to their list of known or suspected sources of spam. Other blacklists use spam trap networks and algorithms to monitor and measure email that flows through their systems, ranking the volume, frequency and content of the email messages and tracing the messages back to the source.
Regardless of the method of compiling a blacklist, mistakes are sometimes made, and legitimate sources of email can be added to the list. In turn, the email service providers and spam filtering applications that reference a given blacklist may begin blocking email based on faulty information.
Due to the imperfect nature of blacklists, many blacklist providers offer a way for an email administrator to request “de-listing” – in other words, to ask the company maintaining the blacklist to reconsider, remove the IP from the list, or to provide additional information about how it got listed. Each blacklist provider has their own mechanism for handling these requests. And some blacklist providers have no mechanism to handle correction requests. Sometimes the process of de-listing an IP address is automatic when a de-listing request is received, sometimes the listing expires automatically, and sometimes the blacklist provider requires several rounds of discussion and proof before removing a blacklisted IP address.
You might be wondering, “What else can cause emails to be rejected?” Blacklists are one method that email providers and mail servers might use to evaluate the legitimacy of an email, but there are other methods used as well. Some large corporations maintain their own internal blacklists based on their users’ interactions with emails or on other filtering mechanisms. Additionally, your recipients may use their email application, such as Outlook, to “block” future emails from you or your domain by moving your messages to their junk folder. Some antivirus applications will also scan a recipient’s inbox for suspicious email and occasionally mistake a legitimate email for a phishing attempt.