Validity has launched a new tool called Heatwave, focused on synthetic email warm-ups, and it is worth knowing about even if you never touch a spreadsheet of DNS records.

Here’s the short version: some cold email tools try to fast-track a new domain’s reputation by manufacturing engagement, often called synthetic warming (or warm-up). Instead of waiting for real people to open, click, and reply, they use networks of controlled mailboxes to simulate that activity. It is a bit like buying followers to look popular before you have actually posted anything worth following.

Mailbox providers and reputation tools have caught on. Heatwave is a new blocklist built specifically to spot domains that show signs of synthetic warming, and Validity says it has already flagged over a million of them.

Why marketers should care

If your team works with cold outreach vendors, lead-generation agencies, or outside contractors, they may be setting up sending domains you do not know about. Heatwave gives you a way to check.

You can look up any domain here: https://lookup.validity.tools/

If a domain comes back listed, that is a signal, not necessarily a death sentence. It is still early days, and we do not yet know how heavily mailbox providers will weigh a Heatwave listing when deciding what lands in the inbox versus the junk folder.

What reputation watchdogs think of Synthetic Email Warm-Up

Heatwave isn’t an isolated opinion. Industry anti-abuse group M3AAWG published a formal position in late 2025 calling out deceptive cold email tactics, and it specifically names artificially simulating subscriber engagement as one of the most serious violations of its standards.

Spamhaus takes an even blunter stance: it considers cold emailing to be spam, plain and simple, and lists fake engagement tools and warm-up services among the tactics fuelling that spam’s growth.

In other words, the organizations that build reputation and blocklist infrastructure already view synthetic warming as abusive, not a grey area. Heatwave is best understood as a new enforcement mechanism for a position the anti-abuse community had already reached.

What a listing actually tells you

If your technical team checks a domain and it comes back listed, the result includes a few extra details worth knowing:

  • How long ago it was flagged. Anywhere from under a week to over six months.
  • A relative severity score, out of 100, comparing it to other recent listings. This score resets hourly, so it is a snapshot, not a trend line.
  • Which stage the domain is in. Stage 2 means synthetic warming activity was observed. Stage 3 means the domain also went on to send real cold outreach.

One quirk worth flagging to your team: a listing does not disappear just because the synthetic activity stops. Validity treats it more like a permanent note in the domain’s file, though there is a process to dispute a listing that was made in error.

The takeaway

Real sender reputation comes from real people responding to real emails. It cannot be manufactured through a network of mailboxes talking to each other.

If you outsource any part of your outreach, ask the vendor directly: how are you warming up this domain? If the answer is vague, that is worth a second look.