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The Dreaded Blocklist and How to Tell if You’re on One

Samantha Orr

Sad Mac
Working in email marketing you’ve probably heard of blocklists and how important it is to stay off of them. But what are they exactly?

“[Blocklists are] Lists of IP addresses that have been reported and listed as “known” sources of spam. There are public and private blacklists. Public blacklists are published and made available to the public—many times as a free service, sometimes for a fee. There are hundreds of well-known public blacklists.”
— Return Path

ISPs have their own protocols for judging an IP and adding it to their blocklists, also known as a blacklists. ISPs often use other reputable lists for blocking an IP. 


Types of blocklists

There are two main types of blocklists. Not all blocklists have the same effect on your delivery. For example, being blacklisted on Proofpoint will primarily affect your delivery to mac.com, icloud.com, and me.com domains. Whereas being listed with Cloudmark or Spamhaus would lead to delivery issues across most major ISPs.

  • IP-based blocklists
    Return Path’s Reputation Network Blacklist
    Spam Cop
    Spamhaus — pbl.spamhaus.org; sbl.spamhaus.org; xbl.spamhaus.org; ubl.unsubscore.com
    ROSKO — Register of Known Spam Operations, run by Spamhaus
  • Domain-based blocklists
    Spamhaus — Dbl.spamhaus.org


Actions that lead to blocklisting

  • List mismanagement. 99% of the time this is the culprit. Sending to hard bounces, unsubscribes, abuse complaints, and known spam traps is a recipe for blocking.
  • Content. This isn’t usually a problem these days, but if you’re using a lot of unusual characters or sending one large image, you can run into issues. Using shortened URLs is a big one here (stay away from these in your email!). Many corporate email systems will conduct filtering to catch emails with spammy content. Emails can either be filtered to the spam folder or blocked by the email server entirely.
  • Infrastructure. Most ISPs are looking for SPF and DKIM records on your DNS. Make sure you have them and that they are set up correctly.
  • Day of the week. Research from Return Path indicates that Wednesdays and Fridays lead to more blacklistings between Spamhaus and CBL.
  • Sudden increase in spam complaints. Spam Cop monitors spam complaints coming in from an IP and will list an IP if there are too many complaints. Many ISPs use Spam Cop to judge the reputation of an IP.
  • Sudden increase in sending volume. It’s no surprise that blacklisting happens most easily between November and December with increased email volume for the holidays.


Actions that help prevent blocklisting

  • Maintain your list. After you send a campaign, make sure you remove hard bounces, unsubscribes and abuse complaints from your list right away.
  • Do not send to hard bounces again. When ISPs notice high hard bounce rates they will “flip the switch” and the rest of your emails will be blocked.  You must maintain an extremely clean list if you want to avoid blocking issues.
  • Send a consistent volume of emails.
  • Balance text and image content in your emails.


How can I tell if I’m on a blocklist?

Check your soft bounces results. If you’re seeing a large amount of soft bounces (anything over 2-3% is suspect and definitely over 5%) it’s possible that you have landed on an ISP blocklist. Are you seeing ‘spam block’, ‘mail block’, ‘spam content’? These soft bounces are an indication of being blocked by the receiving ISP.

A typical blacklisting may last between a few hours or a few days, with some lasting even longer. The duration of the blacklist really depends on what caused the issue and whether that issue has been resolved or not.

If you’re on a blocklist, do not panic. Unfortunately deliverability issues are a normal part of the email marketing industry. You will not be blocked forever. We can help you solve this issue. Email us if you think you might be blocked, and we’ll help you through sorting it out.