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Best Practices

How Can Your Mail Server Avoid IP Blocklists?

By Graeme Caldwell | 4 minute read

Problem, Reaction, Solution?

Hosting companies send millions of emails per day on behalf of their customers and it’s imperative that their message flow runs smoothly, without interruptions. If only it were so simple!

Ask any hosting provider to name the top two problems impacting their ability to provide consistent service to their clients and you will often hear, “spammers and blocklists” in response. Spammers annoy everyone, that’s pretty obvious by now. But the tools used to stop them are often a double-edge sword. Blocklists may stop the spammers but they can also stop legitimate mailers from delivering the messages vital to their business.

Even if you could vet every customer to ensure that they adhere to industry best practices and never send spam, you still can’t control if your client’s computer is compromised. All of this means that providers feel relegated to respond reactively instead of proactively; curtailing the problem before it begins. Some providers find themselves blocklisted every week. Getting your network removed from a blocklist can be cumbersome at the least and next to impossible at the most. Listing can last from one day to several weeks but the impact on your business can extend beyond that period; often resulting in angry customers or worse, ex-customers who decide to move their business elsewhere.

Nevertheless, someone once said, “problems are not stop signs, they’re guideposts”. There is a solution available, a solution that means you’ll never have to think about blocklisting again. But before we get to that, here’s a brief history and explanation of blocklisting.

What is a Blocklist?

A blocklist is a list of IP addresses or domains that are known sources of spam; often referred to as DNSBLs (Domain Name System Blocklists). The technology was built on top of DNS and most MTAs can be configured to reject or flag messages which have been sent from a blocklisted IP. It’s important to remember that a DNSBL is a medium and not a specific list or policy. Policy in this context refers to the criteria that a DNSBL applies to decide which senders should be blocklisted and which shouldn’t. The policy also includes criteria for being de-listed and the process for communicating with blocklisted senders.

There are two main types of blocklists; IP-based and domain-based. The IP-based lists include addresses of sending servers that are known spammers. Domain based URI Blocklists (URI DNSBLs) are lists of domain names that appear within the email body. This blocklist will look for the URLs within the body of the email to see if it contains a domain that has been identified as a source of spam.

Paul Vixie created the first blocklist in 1997. He named it the “Real-time Blackhole List,” and created it in response to the severe and persistent increase in spam during the late ’90s and early ’00s. Since 1997, hundreds of other blocklists have popped up to help in the ongoing effort to curtail spam.

Of these many blocklists, each operator will have their own goals and nomination methods. Some use automated systems and some list manually; meaning a real person makes the decision to include an IP on their list or not.

Some de-listing will occur automatically based on pre-set criteria like a time-period (“listing lifetime”) or reduction in complaint reports. Alternatively, some de-listing requires that a real person review the case manually. This makes blocklisting a highly subjective and a sometimes controversial subject.

The Solution

As we stated earlier, problems are not stop signs, they’re guideposts. If blocklisting is losing you time, money and customers; your network may contain spammers and compromised accounts. MailChannels can rid your network of intentional spammers and identify your comprised customers so that you can reach out to help them secure their system.

We offer powerful outbound spam filtering features that eliminate blocklisting. If you would like to read about other companies that have deployed MailChannels services to stop blocklisting, check out our case studies. You can also contact us for further information.


Photo by Sharosh Rajasekher on Unsplash

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