Email Deliverability

A Marketer’s Field Guide to Cloudmark Email Filtering

minute read

Overview. Most marketers are not familiar with Cloudmark because they don’t host mailboxes nor do they operate under a recognized domain like or However, they do provide anti-spam, phishing and virus protection to email providers covering over 1.6 billion mailboxes worldwide and are therefore a significant factor in getting your email delivered to your subscribers.

Cloudmark’s carrier-grade messaging security system has been around since 2001. Their growing footprint is made up of small and international ISPs, mobile operators, hosting providers and registrars in 165 countries. Customers include EarthLink, Comcast, Cablevision, Time Warner, AT&T, Virgin Mobile, Sprint, British Telecom, Swisscom, Antel, QQ,, WebCentral and many others.

How it works. Cloudmark offers an extensive number of software products and services to providers and you can find their filtering technology at almost all levels of an email messaging system including the edge of the network or gateway, at the MTA or server level and at the desktop level. Cloudmark’s anti-spam technology is made up of three primary components:

1. Global Threat Network – a reputation network made up of ISP abuse teams, system administrators, webmail end-users, desktop users and honeypot spam trap networks.
2. Advanced Message Fingerprinting – real-time algorithms that create fingerprints, or hashes, of various components of an email header, body and other characteristics.
3. Cloudmark Services and Trust Evaluation System – a sophisticated system that collects, analyzes and classifies fingerprints received through its Global Threat Network.

Spam filtering. Cloudmark looks at IP reputation at the edge of the network or gateway. Content (which can also include IP data) is analyzed separately to make additional filtering decisions.

IP reputations are built with input from its Global Threat Network and are therefore built up over time, and will expire over time. If your IP has been blocked at the gateway then you will receive an SMTP error code from the ISP that has implemented Cloudmark CSI (Cloudmark Sender Intelligence) filtering.

Content reputation is very dynamic and can change minute to minute based on real-time feedback from Cloudmark’s Global Threat Network. All messages passing through Cloudmark’s system are ‘fingerprinted’. An individual email can contain many fingerprints, as Cloudmark’s complex algorithms look at all of the parts of a message, including the header, body, and attachments. Anything that might be consistent across multiple emails may generate a fingerprint. Fingerprints can be made spammy or legitimate based on input from the Global Threat Network and will determine whether or not your email gets delivered to the inbox or relegated to the ‘bulk’ folder.

The key take-away here is that if your IP or content is tagged as spammy, you will experience deliverability issues at almost all of Cloudmark’s 1.6 billion mailboxes. Poor email practices are compounded exponentially!

Building a positive reputation. Cloudmark supports legitimate senders and they promote the following email best practices:

• Use dedicated IPs and segment marketing email from transactional email
• Use consistent branding, sending domains and call to action domains
• Utilize feedback loop data to improve your email program and practices
• If you are a low volume sender you should send “heartbeat” messages to keep users engaged and to minimize unknown user rates and spam traps
Follow MAAWG Sender Best Communications Practices (PDF)

Practices that will adversely affect inbox placement rates in Cloudmark’s network include:

• Email appends
• List rentals and purchases
• Mailing to inactive users
• Distributing your mail loads across a large number of IPs and providers
• Un-policed affiliate programs who abuse your content or domain

Cloudmark utilizes Return Path’s Certified Whitelist. You can see how extensive their footprint is by the number of Return Path certified partners who have the Cloudmark logo listed next to it. Participants in this program will experience a lift in reputation and inbox placement at Cloudmark mailboxes.

Support. If you feel that you have been incorrectly classified by Cloudmark then you should contact the affected ISP for support. The mailbox providers own the relationship with Cloudmark and are ultimately responsible for troubleshooting your issues.

Looking for more information on reaching the inbox? Download our Marketer’s Field Guide to Gmail,, and Yahoo.