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How and why ISPs block emails

ISPs use a variety of tools to process and filter incoming email. The spam-blocking measures they use can also result in blocking mail from legitimate senders. This article is an overview of some of the methods used.

About ISPs

ISP stands for Internet Service Provider. When we're talking email marketing, ISP refers to the major email providers: AOL, Hotmail, Outlook, Yahoo, Gmail, Comcast, and so on.

Their customers are commonly your email recipients. It's the job of the ISP to protect their customers receiving unwanted or unsolicited emails.


Blacklists are databases that contain lists of sender domains and IP addresses of known and suspected spammers. Unfortunately, these blacklists also contain many legitimate email service providers (ESPs) and sender domains.

An IP address is a unique number assigned to each computer connected to the Internet. This number is used to identify the source of an email.

Just a few spam complaints can land an IP address on a blacklist, even if the ratio of complaints to the volume of email sent is very low. As an ESP, if we appear on a blacklist, we contact the ISP or blacklist owner and work with them directly to solve the problem.

There are thousands of blacklists in operation around the world, including privately managed blacklists. Below are two very reputable blacklists that have a lot of influence with the major ISPs.


This blacklist adds IP addresses to its list based on the ratio of spam complaints to volume of email sent. An IP address can be added and removed several times even during a 24-hour period depending on the frequency of sampling by SpamCop.

All email marketing, even fully confirmed opt-in mailings, generate some complaints. So sometimes, generally for short periods of time, we can be listed on SpamCop. We monitor this carefully and, if a particular customer is causing the complaints, we either help them clean up their lists, or, if they don't have a permission-based list, we terminate their account.


This is a popular and free blacklist used by ISPs and corporate networks worldwide. SpamHaus also runs ROKSO (Register of Known Spam Operations) that lists spammers who have been terminated three or more times by ISPs.

Spam filters

Another method for blocking is called spam filtering. ISPs block emails based on their content. If there are "spammy" words or phrases used anywhere in the email, including the subject line, a filtering system may block the email. This type of blocking occurs on a per-email basis and is typically used by corporate networks.

Custom blocking

It is common for ISPs and corporate networks to create their own custom set of criteria for blocking. Many ISPs will use information from blacklists and content filters in a "weighted" system that gives "spam points" for each offensive piece of the message.

If the incoming email has spam points above a set threshold it will be tagged as spam and sent straight to the mailbox trash folder, or it may be bounced back to the sender.

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