ISP stands for internet service provider. When talking about email marketing, ISP can refer to any of the major inbox providers: Gmail, AOL, Hotmail, Outlook, Yahoo, Comcast, and so on. Their customers are commonly your email recipients.

It’s the job of the inbox provider to protect its customers from receiving unwanted or unsolicited emails. To that end, inbox providers use a variety of tools to process and filter incoming email. The spam-blocking measures that they use can also result in blocking mail from legitimate senders. Read on to learn more about some of the types of blocklists and other methods of blocking so that you know what to look out for.

Blocklists

Sometimes referred to as “blacklists”, blocklists are databases that contain lists of sender domains and IP addresses of known or suspected spammers. 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 blocklist, even if the ratio of complaints to the volume of email sent is very low. As a result, these blocklists can sometimes contain legitimate email service providers (ESPs) and sender domains.

Because we are an ESP, if we appear on a blocklist, we contact the ISP or blocklist owner and work with them directly to solve the problem.

There are thousands of blocklists in operation around the world, including privately managed blocklists. The two following websites host very reputable blocklists that have a lot of influence with the major ISPs.

SpamCop

SpamCop is a blocklist that adds IP addresses to its list based on the ratio of spam complaints to the 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 contact lists, or, if they don’t have a permission-based list, we terminate their account.

SpamHaus

SpamHaus is a popular and free blocklist 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 inbox providers.

Spam filters

Another method for blocking is called spam filtering. Inbox providers 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 blocklists 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.