A Beginners Guide to Email Blacklisting

Blacklisting is a term that applies to the practice of identifying spammers based on the IP’s and domains that they send email from, then compiling this information into a list that can be referenced by email servers, which then blocks email from those IP’s being delivered to the intended recipient. This is an essential part of maintaining email as an effective tool, however based on the fact that blacklists operate on a largely independent and automated basis, they often mistake good senders for bad.

There are many specific reasons a blacklist may mistake a good sender for spam, but simply put, senders do not pay enough attention to their email marketing approach. If the only focus is to send email campaigns without understanding possible undesired effects, you’re potentially (albeit unintentionally) triggering red flags and causing delivery issues. When properly managed, this risk can be entirely mitigated. This guide will get you on the right track when it comes to understanding the main concepts at play, and beginning to correct any bad email sending practice you currently have.

Email blacklists explained:

Internet Service Providers (ISPs) use many tools to process and filter incoming email. Simply put, these organisations monitor all IPs and domains that actively send email and create a “cheat sheet”, which ISPs can refer to when evaluating an email sender. This is an important mechanism to protect us from receiving spam. Which is good news for those with an overflowing inbox. When sending an email, your IP address, which is like the return address on a letter’s envelope, is logged. This enables mail servers to understand who is sending an email, and also to check if that sender has a history of sending good or bad email, by referencing these cheat sheets, or rather blacklists. A basic process:

  1. You send an email from [email protected] to Joe Soap and your IP address is;
  2. Joe’s mail server takes your mail server’s IP address, resolves that into a domain name, and programmatically checks both the domain and IP against a collection of blacklist algorithms.
  3. If the IP address (or the domain mail.com) is blacklisted, your email is most likely landing in Joe’s junk mail folder, without its content being scanned. And if your IP or domain is flagged as a serial offender, ISPs will reject your emails automatically, meaning it won’t even reach Joe’s junk mail folder.

email-flow (1) As you can see, there is a lot that goes in to determining your emails inbox status. Even if you do strictly follow email marketing best practices, you are at risk of being blacklisted inadvertently. Blacklists can frequently mistake legitimate senders as spammers due to their email sending habits.

Here are some common triggers that lead to blacklisting:

  • Usually, legitimate senders will build their email lists over time and email volumes will increase gradually. A sudden surge in email sending volume indicates possible spammer behaviour.
  • Spammers are some of the world’s most active email senders. If sending very high volumes of email, the potential to be spotted on the spam monitoring radar increases as the general recipients responses increase.
  • High spam complaint rates. Everyone involved in email marketing will receive the odd complaint –e.g. recipients may not recall opting in to receive your messages – however if the number of complaints are above the accepted threshold, ISPs may start routing your emails to spam folders or blocking your emails entirely.
  • Poor email list hygiene can also land you on email blacklists. Sending messages to dormant or inactive email addresses that have been turned into recycled spam traps, or worse, harvested addresses that are actually pure spam traps, is detrimental to your sender reputation
  • Bad content. Certain words or phrases express spam-like content such as “free”or “Money back offer guaranteed”, as well as lots of red colour in the body text, CAPS LOCK, lots of exclamation marks!!!!! etc. are all things that should be avoided.

Your Email Service Provider (ESP) should be able to assist you with both monitoring of blacklists on the web and any advice needed on volumes, frequency and content.

Manage your email reputation and significantly reduce complaints by:

  • Completely dismissing the idea of using a purchased or third party list, and instead build your own email lists in a way that’s going to really give you a return on your investment;
  • Including an unsubscribe link;
  • Regularly removing old email addresses;
  • Sending content that is relevant to the recipient;
  • Avoid sending emails too often.

Doing everything right and still on a blacklist?

Even though you may be following best email practices to a tee, it does happen. The good news is that you can perform email reputation management by monitoring the blacklists on the web. Your ISP should be doing this on your behalf, however, if you’re keen to check this out yourself, you can go to http://multirbl.valli.org/ – a basic tool to monitor whether you are blacklisted. Simply type in your domain or IP and this free multiple DNS Blacklist service will cross reference other blacklists by IPV4, IPV6, or domain.

If you are blacklisted, but believe you shouldn't be, or you’ve addressed the reasons that got you on to the list in the first place, you’ll need to take action to get your IP removed from the list. You can do this by applying for a “delisting” from the blacklist in question by contacting the blacklists’owner via their website.

The key is to be proactive and address these issues quickly to prove to the ISPs that you’re serious about your reputation and about remedying your delivery challenges. If you’re unsure, it is worthwhile to talk to your ESP about it.

Bear in mind, when applying for a delisting:

  • If you are a serial offender, you will find delisting becomes increasingly difficult, if not impossible.
  • Don’t attempt to argue with or get information from listing services. Rather focus on the facts and the remedies you put in place.
  • Applying for delisting when you haven’t fixed the cause, e.g. receiving too many spam complaints, is a bad idea. Fix first and apply later.

Reputable blacklists to keep on your radar: There are copious amounts of blacklists in operation around the globe, including privately managed blacklists. However, not all of them operate the same and some have more influence than others.

Here is a quick overview of some of the more prominent blacklists:

  1. Spamhaus provides real-time anti-spam protection for Internet networks worldwide. It is one of the most reputable blacklisting companies and being listed with them has the most authoritative influence on email delivery.
  2. SURBL is a second tier filter that works in conjunction with Spamhaus. This is a reputable blacklist and effectively indexes spam.
  3. Barracuda Reputation Block List is a free DNS-based Blackhole List (DNSBL) of IP addresses known to send spam and blocks bot-based spam effectively.
  4. Invaluement an example of a less effective blacklist that is now largely ignored by mail server admins.
  5. Spamcop is an effective human-based complaint aggregator that lists IP addresses that SpamCop users report as spammers.

Now that you know more about email blacklisting, you’ll agree that basic email reputation management is one you don’t want to neglect. Although your ESP will always monitor this for you, and make you aware of this (should it ever happen to you), your ESP cannot stop ISPs from receiving negative responses from your recipients in your database.

It is up to you to maintain your brands reputation, and developing an effective marketing strategy on a foundation of email marketing best practices will prevent you getting blacklisted in the future.