Validity Launches Heatwave Blocklist to Combat Unethical Cold Email Practices and Synthetic Domain Warming

Built from millions of targeted data signals, Heatwave helps mailbox providers and ESPs identify manufactured send reputation through fake engagement.

BOSTON, Sept. 3, 2026 /PRNewswire/ -- Validity, the leading provider of Enterprise AI solutions for digital marketers, announces Validity Heatwave, a new email blocklist that identifies unethical cold email practices including artificial domain warming. 

Drawing on an analysis of millions of targeted data points from the Validity Intelligence Network, Heatwave has already listed more than 1 million domains exhibiting the behavioral patterns of unethical email warming.

To query the Validity Heatwave list, visit lookup.validity.tools.

Deceptive domain warming services are increasingly used to build and maintain artificial trust with mailbox providers, clearing a path for cold outreach at scale. They connect to a customer's email tenant, register domains resembling the customer's own, and generate synthetic messages, opens, clicks, and replies between networks of fake accounts — feeding false engagement signals into the systems mailbox providers like Gmail and Outlook use to judge whether a sender is legitimate.

"Email works because people trust it, and that trust depends on the integrity of the signals behind the inbox," said Tom Bartel, Senior Vice President of Data at Validity and board chair of M3AAWG. "When reputation is manufactured instead of earned, it's just plain cheating, and the whole ecosystem suffers. Senders who play by the rules lose out, and mailbox providers end up tightening the screws on everyone. Heatwave gives the ecosystem another signal to identify unethical gaming behavior and act on it."

Not every sender involved in this activity is aware. Because these campaigns often run through subcontracted vendors or partner providers operating several layers removed from the sender's own team, warming activity can happen under a brand's name without anyone at the company knowing. Searching a primary domain at lookup.validity.tools can surface related domains tied to synthetic warming that a sender never knew existed, offering visibility into infrastructure they didn't realize was operating on their behalf.

The Heatwave signal is now mirrored into Validity's existing domain name system (DNS) reputation zones, giving existing message security and mailbox partners already using Validity's reputation data access to the signal without needing to consume it separately.

For ESPs, Heatwave identifies customers who built their sending domain reputation on artificial warming rather than legitimate list development. For mailbox providers, it's an additional signal for filtering decisions on domains whose reputation was synthetically constructed rather than organically earned. Heatwave is used or evaluated by existing global mailbox provider partners, message security partners, including Comcast, Proofpoint, Spamhaus, and SURBL, and ESPs — and the list is growing.

"We utilize Validity reputation data signals," says Alex Brotman, Senior Engineer at Comcast. "This particular signal is interesting because reputation relies on senders intentionally behaving ethically and doing the right thing — nobody likes shortcuts, and nobody likes deception."

"Attempts to manipulate email reputation are not new. We have dealt with automated abuse in many forms for decades, and synthetic engagement is another iteration of the same problem," says Carel Bitter, CEO at Spamhaus. "Our work with Validity on Heatwave gives us an additional signal we can use to correlate domains and better understand the infrastructure behind this activity. That added context helps us identify patterns earlier and respond more precisely."

"We've been applying our domain reputation work to deceptive cold email infrastructure for some time, and Heatwave adds a complementary signal built from a different data set," said Raymond Dijkxhoorn, CEO at SURBL. "The more independent sources pointing at the same domains, the harder it becomes for that deceptive infrastructure to hide."

"Synthetic domain warming is a direct violation of beehiiv's Acceptable Use Policy. Manufacturing fake engagement to trick an inbox into trusting a sender isn't legitimate, it's deception," said Mariska Calabrese, VP of Security at beehiiv. "The senders on our platform earn their reputation the real way, through subscribers who actually want their content. Signals like Heatwave help us and the rest of the ecosystem hold that line, protecting the senders who play it straight from being crowded out by the ones who don't."

Heatwave is part of Validity's ongoing work with organizations across the email ecosystem to address practices that undermine trust in email. Validity participates in industry standards bodies and anti-abuse initiatives, including M3AAWG, ETIS, Signal Spam, ESPC, and BIMI, and works with mailbox providers, ESPs, and other industry partners on email reputation and deliverability.

"For more than 25 years, Validity has been committed to being a force for good in email," said Mark Briggs, Founder and CEO of Validity. "We have visibility into the email ecosystem that nobody else has, built on a data network we've spent decades developing, and partnerships across the industry that put that visibility to work. Heatwave is that commitment in action — using both to make email safer and more trusted for everyone."

The current Heatwave blocklist can be accessed at lookup.validity.tools. ESPs, Message Security, and Mailbox Providers who want full access can contact [email protected].

About Validity

Validity created Engage, the Enterprise AI platform behind the world's most successful email programs. For nearly 30 years, Validity has operated at the center of the email ecosystem—shaping how it works, setting the standards others follow, and earning the trust of the world's biggest brands and mailbox providers. 

That trust is built on the world's largest email data network: proprietary signals from ISPs, exclusive mailbox provider partnerships, and more than 2.5 billion behavioral data points processed daily. This data-driven foundation powers Validity Engage, the Enterprise AI email platform for digital marketers that helps enterprise teams know what to send, when to send it, and who's ready to receive it—so every send is smarter than the last. For more information, visit validity.com or connect with us on LinkedIn.

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SOURCE Validity