About blocklists

Article author
Kelly Sutter
  • Updated

If you’re seeing a large number of soft bounces (anything over 5-10%) over an extended period of time, it’s possible that you have landed on an ISP blocklist, also know as a deny list. Do not panic. Delivery issues are (unfortunately) a normal part of the email marketing industry. You will not be blocked forever.

A typical blocklisting may last between a few hours or a few days, with some lasting even longer. The duration of the block really depends on what caused the issue and whether that issue has been resolved or not. Even if the issue is resolved, bad sending behavior can get you blocked again. Make sure you are following deliverability best practices.


Types of blocklists

Not all blocklists have the same effect on your delivery. For example, being blocklisted on Proofpoint will primarily affect your delivery to Apple domains. Whereas being listed with Cloudmark or Spamhaus would lead to delivery issues across Comcast and other ISPs.

IP-based blocklists SpamCop, Cloudmark, Proofpoint, Spamhaus
Domain-based blocklists Spamhaus


What to do if you think you are being blocked


Step 1: Check if your domain or sending IP is blocked

This tool will test your mail server IP address against 100+ DNS-based email blocklists. The most common ones are listed above.

MX Toolbox Check


Step 2: Help stop this from happening again

If you’ve landed on a blocklist, there are generally four places to start looking for the solution. The most common blocking issues are due to list management issues.

List management 99% of the time this is the culprit. Blocks are a sign that you should clean up your lists. Are you sending to addresses and domains that no longer exist? Are you sending to mailboxes that are full? They may have been abandoned. Make sure you are removing hard bounces, unsubscribes, and abuse complaints right away. (WordFly does this for you.) Do not send to hard bounces more than two times. You must maintain a clean list if you want to avoid blocking issues. 
Infrastructure Do you have DKIM set up correctly for your sending domain? ISPs require this important TXT record on your domain to authenticate your mail.
Sending Volume Are you sending 50 emails one day and 2 million the next? ISPs don’t like inconsistent sending volumes. Also, it is extremely important to slowly warm up your sending when you’re a new sender or when you have switched email service providers.
Email Content This isn’t usually a problem nowadays but some content can still cause issues. Stay away from using shortened URLs, invalid URLs, or sending one large image.