A warmup network is a pool of mailboxes that email each other automatically: your new inbox sends a few messages a day to other members, they open them, pull them out of spam, mark them important and reply, and the volume ramps over two to four weeks. The theory is that Gmail and Outlook see a sender with real engagement. The product was a standalone business in 2020–2022; in 2026 it is a free feature of Instantly, Smartlead and Salesforge, and the standalone vendors charge $10–49 per inbox per month to people outside those tools.
Two facts frame the layer. First, the size claims have become an arms race — Instantly says "over 1,000,000+ real email accounts", Warmy "over 1 million accounts". Second, the question of whether mailbox providers detect and punish these pools is unresolved, and in September 2026 a respected ESP published a post telling senders to avoid warmup services altogether. The full argument is on Does warmup work?.
How the networks work
| Mechanic | What it does | Who describes it |
|---|---|---|
| Peer-to-peer exchange | Your inbox sends to pool members; they reply | All vendors |
| Spam rescue | Pool members move your message from spam to inbox | Standard in Instantly, Smartlead, lemwarm |
| Read emulation | Bots open and scroll the message to mimic dwell time | Instantly: "a private network of headless browsers" |
| Slow ramp | Start at 2/day, add 2/day | Instantly: 2, 4, 6 a day |
| Tiered pools | Better-reputation pools for higher plans | Smartlead: Base, Pro, Smart, Prime, "Ultrapremium" |
| Seed engagement | Real consumer seed mailboxes engage with your mail | Warmy: "500,000+ American seed mailboxes" |
| Health checks | Blacklist and SPF/DKIM/DMARC monitoring alongside warmup | MailReach, Warmforge, Inboxkit |
Pools are only as good as their members. A pool full of fresh cold-email inboxes is a pool of low-reputation senders vouching for each other; that is why vendors segregate "premium" pools and why MailReach stresses a "network of high-reputation accounts" (MailReach).
Who sells it and for how much
| Tool | Model | Price (as of date shown) | Network claim (vendor) |
|---|---|---|---|
| Instantly | Bundled, unlimited | Included in plans (Oct 2026) | "1,000,000+ real email accounts" |
| Smartlead | Bundled, pool tier by plan | Included in plans (Oct 2026) | "35,000+" per Marketbetter review |
| Warmforge (Salesforge) | Per slot; unlimited with Salesforge | $10/mailbox/mo, one slot free (Oct 2026) | not stated |
| lemwarm (lemlist) | Per inbox | $29 Essential / $49 Smart (Jun 2026, per MailReach) | "20,000+ healthy domains" |
| MailReach | Per inbox, includes spam tests | $19.50/mailbox/mo (Oct 2026) | "high-reputation accounts" |
| Warmy | Volume-based | From $49/mo (Aug 2026) | "over 1 million accounts" |
| Warmup Inbox | Per inbox | $15–99 per Marketbetter (Aug 2026) | "30,000+ real inboxes" |
| Inboxkit | Add-on to mailbox | $3/mailbox/mo (Oct 2026) | not stated |
The cleanest market price for warmup is the gap between fresh and pre-warmed mailboxes: Instantly $5 vs $10, Smartlead SmartSenders $4.50 vs $9. Buyers pay about $4.50–5 per inbox per month to skip two to four weeks of ramp — and nothing extra for warmup itself once they are inside a bundling sequencer.
Network-size numbers are not comparable. Instantly counts "accounts", lemwarm counts "domains", Warmup Inbox counts "real inboxes", and none are audited. Smartlead's 35,000 figure comes from a third-party review, not Smartlead.
The economics of the layer
Warmup was a real business when sequencers did not include it: per-inbox pricing at $15–49 against near-zero marginal cost. Bundling destroyed that. Instantly made "unlimited warmup" part of its flat plan; Smartlead put warmup on every tier; Salesforge gives Warmforge unlimited slots to its users (Warmforge). The standalone players have responded by moving up to deliverability suites — MailReach bundles spam-test credits, Warmforge and Smartlead sell Inbox placement testing tiers ($39–$169 and $49–$599 a month respectively), Warmy pitches seed-list engagement.
For an agency with 1,000 inboxes, a $19.50 standalone tool would cost $19,500 a month. No agency pays that; they use the sequencer's free pool. Standalone warmup survives on teams using a sequencer without a pool — HubSpot, Outreach, Apollo users, or Founders and small B2B teams sending from a single domain.
The risk the layer carries
- Detection. Resend's 30 September 2026 post says Validity's "Heatwave" identifies synthetic warming and shares listings with Comcast, Proofpoint, Spamhaus and SURBL. I could not confirm Heatwave on Validity's site (see Does warmup work?).
- Coordinated-behaviour signals. ColdSire, an infra vendor, lists "rapid warmup that looks suspicious" among the triggers for Google suspensions.
- Pool contamination. If your inbox shares a pool with a spammer, its "positive" engagement comes from a bad neighbourhood. Vendors manage this by tiering pools — which concedes the point.
- Terms of service. Automated opens and spam-folder rescues are synthetic engagement. Neither Google nor Microsoft has published a rule naming warmup tools, but both reserve the right to act on "deceptive" behaviour; see Provider rules: Google, Microsoft and the ESPs.
What this means for an entrant
- Ship warmup as table stakes, not as a product. It is free inside Instantly, Smartlead and Salesforge. A sequencer without it fails the agency checklist; a sequencer that charges for it loses the comparison. See Built-in warmup.
- Rent a pool before you build one. A network needs tens of thousands of members to be credible; you can integrate a third-party pool or let customers bring MailReach/lemwarm via API in the first year.
- The opening is honest measurement. Placement tests, seed panels, Postmaster/SNDS data and reply-rate tracking are where the standalone vendors went for a reason. "We tell you where your mail lands" ages better than "our bots open your mail". See Inbox placement testing.
- Position for the post-warmup world. If Heatwave-style detection spreads, the sequencer whose ramp logic uses real sends to real prospects at low volume — and pre-aged inboxes from clean suppliers — wins. Build the ramp so warmup can be switched off without losing deliverability.
- Watch the pre-warmed price. At $4.50–5/inbox/month of premium, pre-warmed inventory is a real margin line for whoever bundles infra; see Inbox & domain infrastructure.