What Happens When the Email Validation Company Starts Blacklisting the Warming Tactics You've Been Told to Use?
The very tool that has been the gatekeeper of email reputation for years is now turning its sights on the domain warming industry itself. Validity — the company whose data feeds mailbox providers, ISPs, and ESPs — just launched Heatwave, a blocklist specifically designed to catch senders who are using automated, synthetic warming to game their way into inboxes. If you have been paying a third-party service to "warm" your cold domains by generating fake opens, clicks, and replies, you are now at risk of being publicly listed — and that listing could destroy your deliverability overnight.
This is not a theoretical threat. Heatwave has already listed more than 1 million domains. Spamhaus has endorsed it. Comcast, Proofpoint, SURBL, and beehiiv are either using or evaluating the signal. The same infrastructure that validates your email reputation is now actively policing the shortcuts that many cold emailers have come to rely on.
Why Artificial Domain Warming Was Always a House of Cards
Domain warming is a legitimate practice: you start a new domain, send low volumes of highly targeted emails to engaged recipients, and gradually increase volume as mailbox providers learn to trust you. The problem is that this takes weeks or months. So "warming services" emerged, promising to compress that timeline by creating networks of fake accounts that generate synthetic engagement signals — opens, clicks, replies — all designed to trick Gmail, Outlook, and others into thinking the domain is trusted.
Tom Bartel, senior vice president of Data at Validity and board chair of M3AAWG, put it bluntly: "When reputation is manufactured instead of earned, it’s just plain cheating, and the whole ecosystem suffers." With Heatwave, Validity is putting real teeth behind that statement. The blocklist scans primary domains and surfaces related domains that fuel synthetic warming. If your domain is connected to a warming network — even if you subcontracted it to a vendor three layers removed — Heatwave can find you.
How Heatwave Works: The Technical Reality
Artificial warming systems work by registering domains that resemble the sender’s own domain, then generating synthetic messages, replies, opens, and clicks between networks of fake accounts. These false engagement signals flow into mailbox provider systems. Senders often don't know it’s happening because they outsource warming to subcontracted vendors. Validity's Heatwave connects those dots.
The Heatwave signal is mirrored into Validity’s existing domain name system reputation zones. Any mailbox partner that already uses Validity’s reputation data can access this signal without additional integration. That means your sender reputation could now be downgraded not just for spam complaints or bounces, but for any association with synthetic warming networks.
Raymond Dijkxhoorn, CEO at SURBL, said: "The more independent sources pointing at the same domains, the harder it becomes for that deceptive infrastructure to hide." Heatwave adds a complementary signal built from a different data set — one that specifically targets the warming infrastructure itself.
The Scale Is Already Alarming
Validity reports that Heatwave has listed more than 1 million domains. That’s not a pilot program. This is a live blocklist that is already being fed into mailbox provider decision engines. If you run cold email campaigns, you need to check whether any of your domains — or domains used by your vendors — appear on this list.
Carel Bitter, CEO at Spamhaus, stated: "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." Spamhaus’ endorsement means Heatwave will likely become a standard signal used by the broader email ecosystem. Domains listed on Heatwave may soon find themselves blocked by multiple inbox providers simultaneously.
Why This Matters More Than Any Previous Blacklist
There have always been blocklists for spam, for phishing, for malware. But this is the first major blocklist specifically targeting the warming infrastructure that cold emailers depend on. The irony is obvious: the same services that promised to help you build reputation are now exactly what will destroy it.
Mariska Calabrese, vice president of security at beehiiv, said: "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. 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."
beehiiv is not an isolated case. Comcast, one of the largest ISPs in the U.S., is already using the signal. Alex Brotman, senior engineer at Comcast, commented: "We utilize Validity reputation data signals. 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."
The message is clear: mailbox providers are tired of being gamed. They are now actively partnering with Validity to identify and penalize artificial warming.
What You Need to Do Differently Starting Today
If you are running cold email campaigns, you need an honest audit of your warming practices. Here is concrete advice:
- Stop using any third-party warming service that generates synthetic engagement. If you are paying for opens, clicks, or replies from fake accounts, you are now a target. Verify whether your vendor creates real, engaged subscribers or simulated activity.
- Audit your domain infrastructure. Validity’s Heatwave looks not just at your primary domain but at related domains that may be used for warming. If you have subdomains or lookalike domains registered by a warming vendor, check them immediately. A single linked domain could get your entire sending infrastructure blacklisted.
- Switch to organic, content-driven warming. Domain warming does not have to be artificial. Start by sending real, personalized emails to a small list of recipients who actually want to hear from you. Gradually increase volume based on engagement signals like genuine replies and click-throughs. This takes longer, but it builds reputation that mailbox providers respect — and that Heatwave cannot flag.
- Monitor Validity’s reputation data. If you use an ESP or email platform that integrates with Validity, ask whether they have incorporated the Heatwave signal. If you send directly, consider signing up for Validity’s reputation monitoring service or use a third-party tool that checks against this new list.
- Reassess your cold email strategy entirely. The days of blasting warmed domains with generic templates are ending. The senders who will survive are those who invest in list quality, personalization, and genuine value. If your campaigns rely on automation that mimics human behavior, you are gambling that Heatwave will not catch you.
The Unresolved Tension: What About Legitimate Warming?
Here is the uncomfortable question that Validity has not yet answered: where exactly is the line between legitimate, organic warming and "artificial" warming? Every sender starts with a cold domain. Sending targeted emails to a small, engaged list is fine. But what if you use a tool that automates the sending schedule? What if you use an email sequence that looks manual but is actually automated? At what point does optimization become manipulation?
The industry needs clear guidelines, not just a blocklist. Validity’s Bartel calls synthetic warming "cheating," but many cold emailers believe they are using legitimate automation to accelerate a natural process. The difference often comes down to the presence of fake engagement — but what about services that send to real people who merely consent to receive emails? Is that still warming, or is that just normal sending?
Spamhaus’ Bitter said synthetic engagement is "another iteration of the same problem," but he did not define where acceptable warming ends and abuse begins. That ambiguity leaves senders vulnerable to sudden blocklisting even if they believe they are following best practices.
The Bottom Line for Cold Email Practitioners
Validity's Heatwave is a game-changer. It flips the script: the tools you relied on to build reputation are now the tools that can destroy it. The 1 million domains already listed is a warning shot. If you are using any form of synthetic warming, you are gambling with your deliverability — and the house is now watching the cards.
The smart play is to pivot now. Invest in real list building, real engagement, and real reputation. The window for shortcuts is closing. Will you adapt before your domain ends up on the very blocklist that was meant to protect the ecosystem?