Warm-up is the single most-sold feature in the cold outreach tooling market. Every vendor promises a magic network that teaches Gmail to trust your domain. Most of them were built around a simple trick: thousands of participating mailboxes send each other messages, open them, reply to them, mark them as Important, and rescue them from Spam. The signals flow back to the ISP, and the ISP learns that your domain is legitimate.
That worked well enough from 2018 to 2023. Then Gmail's filtering team started treating clustered sending networks as a pattern in itself. In late 2024 Postmaster Tools started showing sudden reputation drops on accounts that had previously been stable — a lot of them shared the same warm-up vendor.
Warm-up is still mandatory for new domains. But the exact tool you pick now matters more than it used to. Vendors who updated their networks in 2024–2025 are still safe. Vendors who kept the 2019 playbook are now a liability.
How warm-up tools actually work
The mechanics are near-identical across every vendor. You connect your sending mailbox over IMAP/SMTP or OAuth. The service adds your account to a pool of thousands of other connected mailboxes. Every day, the pool generates a volume of emails between members — plain text, innocuous subjects, sometimes lifted from a prewritten corpus. Each message is read, replied to, marked as Important, and pulled from Spam if it lands there.
The sender domain accumulates positive engagement signals. ISPs see a new domain getting consistent opens and replies from real accounts. Domain reputation rises. After 3–4 weeks you can start cold sending with a meaningfully higher inbox rate than you'd have from a cold start.
That was the theory. The pattern that went wrong: the senders and recipients were all members of the same small network, producing correlated engagement patterns that no normal domain would ever produce.
Mailwarm
The original. Mailwarm runs a network of roughly 1,000 curated mailboxes across Gmail, Outlook, Yahoo and a handful of smaller ESPs. You pick a daily volume (50 to 1,000 warm-up emails per day) and Mailwarm handles the rest. Pricing starts at $69/month for a single inbox and scales to $299 for 25.
Post-2024, Mailwarm rebuilt their engagement corpus, randomised send times more aggressively, and pruned low-quality mailboxes from the pool. Third-party placement tests in early 2026 show Mailwarm-warmed domains hitting ~80% Gmail inbox after 4 weeks, versus ~55% for unwarmed controls. Still working, but not the miracle it used to be.
Warmup Inbox
Smaller pool (several hundred mailboxes), heavier overlap with Instantly users. Warmup Inbox positions itself as the cheap alternative at $19/mailbox/month. The cheap price reflects the cost: a smaller, less diverse pool, and less infrastructure to randomise patterns.
In our testing Warmup Inbox still delivers a measurable lift on new domains, but Postmaster Tools reputation recovery on problem domains was noticeably slower than Mailwarm or Mailreach. If you're already in Instantly, it's convenient. If you're not, it isn't worth switching to.
Lemwarm
Lemlist's built-in warm-up. Bundled with Lemlist subscriptions. Lemwarm was one of the first to attempt "personalised" warm-up content — conversations that look more like real SDR replies than generic filler. The personalisation engine was a clear improvement over template-based filler in 2022, but the pattern was public enough that filters caught up.
In 2025 Lemlist rebuilt Lemwarm around a new engagement pattern that samples real-looking conversation fragments and varies send cadence per-mailbox. Early results are back to acceptable — roughly on par with Mailwarm. If you're already paying for Lemlist, there's no reason to run a second tool.
Warmy
Warmy markets itself on aggressive engagement simulation — replies, stars, folder moves, forwards. The sales pitch is that Warmy produces more diverse engagement than competitors. The flip side: more aggressive engagement means more obvious engagement, and engagement is one of the signals filters now look at.
Warmy's 1-click setup is the slickest in the market, and on brand-new domains with no prior history it produces faster ramp than Mailwarm. On domains with existing problems, we've seen it accelerate a reputation drop rather than recover it. Use with caution — especially if you're already on a Medium reputation in Postmaster Tools.
Mailreach
French-built, spun out of a deliverability testing company. Mailreach runs the most selective pool of the major vendors — real mailboxes of real business users, vetted during onboarding. Pricing starts at $25/mailbox/month and scales by inbox count.
Because the pool is smaller and more genuine, engagement patterns look closer to organic. Mailreach-warmed domains consistently score highest in our placement tests. It's also slower — expect 4–5 weeks to hit full ramp versus 3 weeks on Mailwarm. The trade-off is worth it for long-term cold outreach programs.
What Gmail now detects
Google doesn't publish its filter rules, but the pattern from Postmaster Tools data is consistent. Filters are now trained to spot:
- Clustered sending networks where the same group of domains exchanges mail in statistically unusual ways.
- Identical reply templates, especially short replies ("Thanks, got it", "Sounds good") sent across hundreds of unrelated domains.
- Engagement that happens at predictable hourly intervals, or within seconds of delivery.
- Folder-move actions (Spam → Inbox) executed in batches rather than naturally during reading.
- Very high engagement on messages with zero CTAs or attachments.
Every one of those is what a badly-designed warm-up network produces. The vendors who updated in 2024–2025 addressed most of them. The ones who didn't are now a net negative.
Signs your warm-up is hurting you
- Postmaster Tools domain reputation drops from High to Medium without a change in your cold sending volume.
- Sudden appearance on smaller DNSBLs (URIBL, Invaluement) despite unchanged cold outreach.
- Gmail spam complaint rate climbs above 0.1% even though cold send volume is stable.
- Inbox placement in third-party tests is lower mid-warm-up than it was on day one.
If any two of those are true at once, pause the warm-up, let the domain sit for 7–10 days, and switch vendors.
The manual warm-up alternative
For domains where cold outreach is a small part of your overall business, manual warm-up is better than any tool. The process: send genuine business emails — internal team comms, customer conversations, personal outreach to your actual network — at a volume that grows from 10/day to 80/day over 4 weeks. No tool, no pool, no artificial engagement.
The trade-off is obvious: it requires real recipients and a real reason to email them. If you're starting a cold outreach program from scratch with no existing audience, manual warm-up isn't available to you.
What 2026 warm-up should look like
The vendors still producing results share a set of traits:
- Selective pool curation — real users, vetted at onboarding.
- Slow ramp — 10/day to 80/day over 4+ weeks, not overnight.
- Engagement variation — reply timing distributed naturally, not scheduled.
- Conversation-shaped message bodies, not repeated templates.
- Published placement test data — not just warm-up network size.
Warm-up is still necessary for any new sending domain. In 2026 the shortlist that reliably helps is Mailreach, Mailwarm (post-2024 rebuild), and Lemwarm (post-2025 rebuild). Warmup Inbox is fine if you're in the Instantly ecosystem already. Warmy is risky on anything but brand-new domains. Everything else — assume it's hurting you until a current placement test proves otherwise.