Imagine a city in a dummy Silicon Valley with a high population of bad actors (spammers in emailspeak). The city government decides to create a list of all their addresses. The list is then given to city-wide mail carriers. When a carrier sees an address on the list provided by the government, they immediately know not to deliver any mail to that address.
That’s (email) blacklisting in a nutshell. When an email provider blacklists a domain or IP address, it’s basically telling other email providers not to deliver any email from that address to recipients’ inboxes.
Messages from blacklisted email addresses are typically blocked or diverted to the junk/spam folder. In certain cases, emails from blacklisted senders are rejected at the server level, downright preventing entry into the recipient’s email system.
What Is An Email Blacklist?
An email blacklist is a list of senders (IP addresses or domains) flagged as sources of spam, unwelcome messages, or potentially malicious content.
Spam is as old as the internet. Notably, in spite of the creation of the Real-time Black Hole List (one of the first email blacklists) in 1997, it was not until the turn of the century that blacklisting was finally adopted as a means to throttle spam. Prior to blacklisting, mail servers relied solely on manual content filtering and user feedback to identify spammy messages.
Spammers are more sophisticated now. Accordingly, while RBLs are still used today, they’re not the only kind of blacklists.
Types of Email Blacklist
There are several kinds of blacklists, from RBLs to URIs. Beginning with the beginner, let’s take a brief look at them one by one.
- Real-time Blackhole Lists (RBLs): These dynamic blacklists contain IP addresses or domains that are known to be used by spammers.
- Composite RBLs: Composite blacklists contain data from various RBLs and reputation systems.
- Domain-based Blacklists: As the name suggests, these blacklists identify and block emails coming from specific domains known for sending spam.
- IP-based Blacklists: IP-based blacklists track and block emails coming from IP addresses known for sending spam.
- URI Blacklists: Uniform Resource Identifier (URI) blacklists identify and block potentially malicious URLs in emails.
- Community-based Blacklists: These blacklists rely on spam alerts from an email community. These lists are typically used by organizations that depend upon user feedback.
- Proprietary Blacklists: Organizations may contain their own internal blacklists to deal with known bad actors. Criteria for blacklisting will vary from one organization to another.
- Regulatory Backlists: Some government and regulatory bodies may contain blacklists to block messages from known sources of spam.
- Geo-based Blacklists: For national security reasons, governments may use geo-based email blacklists to block messages coming from certain countries or regions.
- Spamtrap Blacklists: A spamtrap blacklist contains domains or IP addresses that have sent emails to spam traps.
- ASN Blacklists: These blacklists contain IP address ranges associated with specific autonomous system numbers or ASNs. ASNs are identification numbers assigned to a group of IP addresses. An ASN blacklist identifies sources of spam based on the network responsible for malicious IP addresses.
- Botnet Blacklists: These blacklists are used for tracking and blocking IP addresses that are part of a compromised network of computers, known as a botnet, that is used for carrying out malicious activities.
Now, let’s look into the specific reasons or scenarios that lead to email addresses being added to blacklists.
Why Email Addresses Get Blacklisted
Here are nine reasons why email addresses may show up on a blacklist. Following this, we will look at how to avoid getting blacklisted and then how to get off an email blacklist.
1. Sender reputation: If a particular IP address has a history of sending low-quality, illegitimate emails, it has a poor reputation, and vice versa. Poor reputation always leads to blacklisting.
2. Email volume: If there is a sudden spike in email traffic, ISPs suspect the sender to be a source of spam since spammers tend to send too many emails in a very short time.
3. Suspicious content: The presence of spammy keywords, potentially malicious attachments and URLs, and explicit content (read: adult content) may trigger blacklisting.
4. DKIM and SPF snafus: DKIM (Domain Key Identified Mail) involves digitally signing emails with a private key. If the signature can’t be verified by a mail server, it’s an indication of illegitimacy.
SPF (Sender Policy Framework) determines which servers are authorized to send emails from a particular domain. Emails from unauthorized servers lead to blacklisting.
5. User-level abuse reports: Email service providers can warn senders whose emails are frequently marked as “spam” by recipients. (Note that user-level filtering doesn’t lead to immediate blacklisting.)
6. Phishing campaigns: Emails that impersonate legitimate organizations and individuals can lead to blacklisting.
7. High bounce rates: Failure to maintain a clean email list leads to invalid addresses creeping into email campaigns, which can trigger blacklisting due to high bounce rates.
8. Legal noncompliance: Not including unsubscribe options as mandated by laws like the CAN-SPAM in the United States can result in blacklisting.
9. Proxy servers and open relays: An open relay is an email server that allows any email to pass through it without requiring authentication. On the other hand, proxy servers tend to mask the sender’s true IP address by delivering emails through their own servers. Both proxy servers and open relays are abused by spammers, which can result in blacklisting.
Finally, let’s understand how a brand can avoid being blackballed by email servers.
How to Avoid Email Blacklisting
Evidently, ensuring legal/regulatory compliance, avoiding spammy language, A/B testing emails, using double opt-ins, list cleaning, warming up new IP addresses, and so on, are some of the well-known blacklisting detours. However, here are five often-forgotten ways to avoid being included among blacklisted email addresses.
1. Monitor URL reputation: Check the reputation of all URLs in your email before hitting Send. Make sure there are no links that lead to malicious or compromised websites. You can use online services like VirusTotal, AbuseIPDB, Cisco Talos, and IP Void.
In addition, be careful when using URL shorteners. Since they obscure the original destination, some email filters may find them suspicious. You want to be transparent about the destination of the links in your email.
2. Social media cross-promotion: Promote your email campaigns on your social media page and ask your followers to subscribe.
As we discussed, recipients can trigger spam filters if they realize they are receiving unwanted emails. But when people willingly subscribe to your newsletter, they are less likely to mark your emails as spam.
3. Leverage text-only emails: Not all of your emails contain, or are meant to contain, promotional content, right? So, feel free to use text-only emails from time to time. They are more direct and personal, hence not so vulnerable to being flagged as spam.
At the same time, don’t send text-dense emails, which are equally suspicious.
4. Monitor blacklist status: Just like regular health checkups, you need to regularly check whether your email address has somehow surfaced on a blacklist. For that, take help from popular online services like Spamhaus and MxToolbox.
5. Educate your marketing team: This is arguably the most neglected point. Avoiding blacklists may seem intuitive. However, bear in mind that spam criteria are updated regularly, thanks to the pace at which spammers update their own techniques. See to it that your team keeps in step with the developments on both sides of the border.
So those were some of the less-discussed ways to avoid being vetoed by mail servers. But if the milk has already been spilled, how to get off an email blacklist?
The very first step is to identify the blacklist. The websites we recommended will help you with that. Next, try and figure out why you got blacklisted. Double-check your email practices. Review your email list and content. Review bounces and spam complaints. In case it’s a security issue, contact your server.
You can also request the blacklist provider for delisting. Some providers do allow that. Visit their website and familiarize yourself with the delisting process. Provide necessary information, reasons for delisting, and actions already taken to implement delisting and initiate the process.
If none of the above works, you may need to seek professional help for email blacklist removal. Contact email deliverability experts and consultants.
No one likes being blacklisted. And sometimes, even robust preventative measures go phut until brands are rightly surprised to find their IP addresses on a blacklist.
Fortunately, there are always remedies available, and the quicker you crush these bugs, the firmer your grasp on the marketing laurel becomes.