Validity launches Heatwave, a blocklist aimed at synthetic domain warming

Validity announced Heatwave on September 3, 2026: a domain blocklist built to identify unethical cold email practices, chiefly artificial domain warming. According to Validity's announcement, the list already includes more than one million domains showing the behavior patterns of unethical warming. Al Iverson examined the launch on Spam Resource on September 9. He described the list as covering both synthetic warming and active cold outreach to real recipients.

What it targets

Validity's announcement describes how deceptive warming services work. They connect to a customer's email tenant, register domains that resemble the customer's own, and generate synthetic messages, opens, clicks and replies between networks of fake accounts. The result is manufactured engagement: the same positive signals mailbox providers use to judge whether recipients want a sender's mail.

Validity pitches the data to two audiences. For ESPs, Heatwave flags customers whose sending domain reputation was built on artificial warming rather than real list growth. For mailbox providers, it is an extra filtering signal for domains whose reputation was synthetically constructed.

Who is using it

The announcement names Comcast, Proofpoint, Spamhaus, SURBL and beehiiv among organizations using or evaluating Heatwave, along with unnamed mailbox provider, message security and ESP partners. Iverson noted that no adoption by Google or Microsoft has been confirmed. Anyone can look up a domain at lookup.validity.tools; ESPs, security vendors and mailbox providers can request full access from Validity. Pricing was not disclosed.

What it means for senders

The line Heatwave draws matters. Legitimate email warm-up means raising volume gradually to real, opted-in recipients. What the list targets is different: networks that manufacture engagement to fake that history. In his write-up, Iverson argues that cold email sent in bulk is unsolicited bulk email, and he points to an earlier M3AAWG position paper condemning deceptive cold email practices.

In practice, if a domain's reputation came from a warming network, that history can now be looked up by some of the organizations deciding whether its mail gets delivered. For senders who have never used such services, nothing changes. Build reputation on consent, such as double opt-in, and keep watching the blocklists that matter with our blacklist checker.