Open LinkedIn or any cold email forum and you will hit the same pitch within minutes: drop your inbox into a warmup network, let the bots send and reply for two weeks, and watch your sender reputation climb. Services like Mailwarm, Lemwarm, Warmup Inbox, Mailreach, and the warmup features built into Smartlead, Instantly, and SalesBlink have built a sizable industry around this promise.
The reality in 2026 is that mailbox providers have caught on. Google's February 2024 bulk sender requirements gave Gmail teeth to enforce against engagement manipulation. Microsoft's May 2025 enforcement of the same standards closed the remaining gap. Major email warmup services still work, mostly, but the risk profile has changed materially and the underlying problem they pretend to solve is now harder to hide.
- Email warmup services rotate messages through networks of mailboxes that auto-reply, mark as important, and pull messages out of spam to manufacture engagement signals.
- Google and Microsoft explicitly classify automated warmup networks as a violation of their bulk sender policies and have started detecting and terminating accounts.
- Mailbox providers detect warmup networks through reply pattern fingerprinting, social graph analysis, engagement velocity anomalies, and IP correlation.
- Legitimate IP warmup and domain warmup using real subscriber engagement remain the only sustainable path to a strong sender reputation.
- If your business model requires automated warmup to function, your underlying sending practices probably will not survive 2026 enforcement either.
What Email Warmup Services Actually Do
An email warmup service is a coordinated network of inboxes that send messages to each other, reply to those messages, mark them as important, move them out of spam folders, and star them. When you plug your inbox into the network through OAuth or SMTP credentials, the service sends a small number of messages from your account to other network members and triggers the same engagement behaviors on incoming network mail.
The premise is simple. Mailbox providers like Gmail and Outlook use engagement signals to determine inbox placement. If a new sending address suddenly shows high reply rates, important markers, and zero spam complaints, the provider concludes the sender is trustworthy and the address gets favorable filtering.
A typical warmup configuration ramps from 5 messages per day to 40 or more over two to four weeks, with reply rates engineered to look organic at 30 to 50 percent. Most services also rotate which network members interact with your inbox to avoid obvious correlation patterns.
The Major Email Warmup Services
The market has consolidated significantly. Five products handle most of the warmup volume in 2026:
- Lemwarm (lempire): One of the original warmup networks, now bundled with Lemlist outreach. Reports a network of several hundred thousand inboxes.
- Warmup Inbox: Standalone service marketed to agencies and high-volume cold senders. Includes detailed daily reports on inbox placement across providers.
- Mailwarm: Manual-style warmup that simulates human send patterns. Limited to Google and Microsoft workspace accounts.
- Mailreach: Combines warmup with placement testing using a network of seed addresses. Popular with deliverability consultants.
- Built-in warmup (Smartlead, Instantly, SalesBlink, QuickMail): Cold email platforms that ship warmup as a free or low-cost feature alongside outreach automation.
Pricing typically runs from $15 to $90 per inbox per month. Agencies often warm 20 to 200 inboxes simultaneously, making this a meaningful operating expense and a meaningful business model for the vendors.
Why Google and Microsoft Consider This a Violation
Google's bulk sender requirements, in effect since February 2024 and tightened through 2025, explicitly prohibit manipulation of engagement signals. The Gmail Postmaster documentation and the related sender guidelines state that senders must not artificially inflate engagement, including through bots, networks of accounts, or coordinated send-and-reply schemes.
Microsoft published equivalent language in its May 2025 sender requirements update for Outlook.com, Hotmail, and Live. The policy targets exactly the behavior warmup services automate: synthetic replies, artificial important markers, and coordinated spam folder rescues.
The reasoning is straightforward. Engagement signals only work as a trust mechanism if they reflect real user behavior. When a warmup network injects fake engagement, it pollutes the data mailbox providers use to protect their actual users. That is not a gray area for Google or Microsoft. It is the exact behavior their spam filters are designed to detect and punish.
Industry context: Google's February 2024 enforcement against bulk senders included language in postmaster communications that automated send-and-reply networks designed to manipulate engagement signals would be treated as a violation regardless of whether the sender met SPF, DKIM, and DMARC requirements. Authentication compliance does not shield you from this category of enforcement.
How Mailbox Providers Detect Warmup Networks
Detection is multi-layered and has improved significantly since 2023. The signals fall into a few categories.
Reply pattern fingerprinting
Warmup replies follow recognizable patterns: short generic acknowledgments, replies sent within tight time windows, replies from addresses that never reply to anyone outside the network, and replies that quote no content from the original message. Modern filters profile these patterns at scale.
Network correlation
When 200,000 inboxes all reply to each other in rotating patterns, the graph of who-emails-whom is statistically distinct from organic correspondence. Gmail and Outlook can analyze the social graph of an account and identify clusters that look like warmup networks rather than real relationships.
Engagement velocity anomalies
A new sending domain that suddenly receives 30 percent reply rates from 50 different mailboxes in week one looks nothing like an organic ramp. Real domain warmup shows gradual engagement growth that correlates with sending volume increases. Synthetic warmup shows a step function.
IP address correlation
Many warmup network mailboxes run from a small pool of cloud IP ranges. When the same handful of IPs are interacting with thousands of new sending domains every month, providers notice.
Header and client fingerprinting
Warmup-generated replies often come from automation tooling that leaves distinct header signatures, missing User-Agent strings, or unusual SMTP behaviors that organic clients do not produce.
The Real Consequences of Getting Caught
The penalties for being detected as part of a warmup network range from mild to catastrophic:
- Engagement signals discounted: The minor consequence. Gmail or Outlook simply stops counting your warmup-generated replies as positive signals, neutralizing the entire reason you paid for the service.
- Domain reputation penalties: Detected manipulation can trigger a permanent mark against your domain reputation at the provider, suppressing inbox placement even for legitimate mail sent later.
- Account termination: Google Workspace and Microsoft 365 have both terminated accounts found to be participating in warmup networks. Termination includes data loss, suspension of related services, and in some cases a block on creating new accounts from the same payment method or recovery email.
- Domain-wide impact: A penalty on one inbox at a domain can affect the entire sender reputation of that domain, harming transactional and marketing mail from unrelated systems.
If you must use a warmup service, never plug your primary domain into it. Use a dedicated subdomain or, better, a completely separate domain for cold outreach. That way, a reputation penalty does not poison your transactional and marketing infrastructure.
What Actually Builds Sender Reputation in 2026
The unsexy truth is there is no shortcut. Real IP warmup is built on the same foundation it has always been built on: send small volumes to engaged recipients, grow gradually, and let real engagement signals accumulate.
A proper warmup ramp for a new domain looks like this:
- Set up authentication first. Publish valid SPF, DKIM, and DMARC records before sending a single message. Verify alignment using a DMARC checker.
- Send to your most engaged real subscribers first. Your top 10 percent of openers in week one, your top 20 percent in week two, and so on. These are real humans who will reply, click, and reinforce positive signals organically.
- Ramp volume gradually. Double sending volume every two to three days, capped by your initial total list size. A domain typically reaches full sending volume in four to six weeks.
- Monitor with provider tools. Watch Google Postmaster Tools and Microsoft SNDS for reputation signals throughout the warmup period.
- Suppress unengaged segments. Do not include cold or unengaged contacts during warmup. They lower your engagement metrics at exactly the wrong time.
For cold outreach specifically, the path forward is harder but real. It involves smaller sending volumes per inbox, more careful list quality through real-time email verification, slower ramp speeds, and content that actually generates real replies. The cold email industry is being forced toward sustainable practices, and the businesses that adapt now will outlast the ones leaning on warmup networks.
Several major cold email platforms quietly removed their warmup features or made them opt-out by default in 2025 after enterprise customers reported account terminations. The vendors that still aggressively market warmup are now mostly the ones serving the highest-risk segment of the market.
When to Walk Away From Warmup Services Entirely
If any of these apply, the warmup service is the symptom, not the cure:
- Your business model requires sending 100 cold emails per day per inbox to be profitable. That volume is incompatible with sustainable deliverability under current bulk sender rules.
- You are warming up the same inboxes month after month because they keep dropping into spam. The underlying content, list, or sending pattern is the problem, not the warmup.
- You operate at a scale that would put you over the 5,000 messages per day threshold for Google's strict bulk sender enforcement, but you are still trying to send from Workspace accounts.
- You are paying for warmup on dozens of domains and inboxes to avoid getting caught. That cost structure is a tell that the underlying activity is borderline.
Legitimate senders do not need to fake engagement. If your warmup spend is a recurring line item, the higher-leverage move is to fix the upstream problem: better list hygiene, more relevant content, lower volume per inbox, or a different sending architecture entirely.
Frequently Asked Questions
Email warmup services are not illegal in most jurisdictions, but legality is not the relevant question. Google Workspace, Microsoft 365, and most major mailbox providers prohibit warmup networks in their terms of service. Using one risks account termination regardless of whether the activity is technically lawful.
Warmup services worked reliably in 2020 and 2021. They worked less reliably in 2023. As of 2026, with active detection at Google and Microsoft, results are inconsistent and the downside risk has grown significantly. Even when they produce short-term inbox lift, the engagement signals are often discounted by modern filters.
Yes. Google has publicly stated that automated send-and-reply networks designed to manipulate engagement signals are detectable and treated as a policy violation. Detection methods include reply pattern fingerprinting, social graph analysis, engagement velocity anomalies, and IP correlation across known warmup network infrastructure.
A new domain typically needs four to six weeks of gradual ramp-up to reach full sending volume safely. Start with your most engaged subscribers, double volume every two to three days, and monitor reputation in Google Postmaster Tools and Microsoft SNDS throughout the process. There is no shortcut that survives 2026 enforcement.
IP warmup is the legitimate practice of gradually increasing send volume from a new IP address while real recipients engage with the mail. Email warmup services are third-party tools that automate fake engagement through networks of bot-controlled mailboxes. The first builds real sender reputation; the second tries to manufacture it artificially.