.svg)
Instantly Email Warmup: How to Build Sender Reputation
You bought the domains. You set up the mailboxes. You loaded your first campaign into Instantly. And then your open rates came back at 8%.
If you've been there, you already know the answer: your sending infrastructure wasn't warmed up. Email service providers like Google and Microsoft don't trust new mailboxes. If you start blasting 200 cold emails a day from a domain you registered last week, those messages are going straight to spam—or getting silently dropped before they ever reach an inbox.
Email warmup is the process that fixes this. It builds your sender reputation gradually by simulating real email activity—sending, receiving, opening, replying—so that ESPs learn to trust your mailbox before you start using it for outreach. Instantly has one of the most widely used warmup tools in the cold email space, and getting it right is the difference between campaigns that land in the primary inbox and campaigns that disappear.
This guide covers how Instantly's email warmup works, how to set it up correctly, the timeline you should expect, and the best practices that separate teams with 60%+ open rates from those stuck in spam folders.
Instantly's warmup tool sends automated emails between real mailboxes in its warmup network, generating opens, replies, and positive engagement signals that build your sender reputation with ESPs. Enable it as soon as you connect a mailbox, warm up for at least 2–3 weeks before launching campaigns, and keep it running permanently alongside your outreach. Most teams see deliverability stabilize around the 3-week mark with proper DNS authentication (SPF, DKIM, DMARC) in place.
What Is Email Warmup and Why Does It Matter?
Email warmup is the process of gradually increasing sending volume from a new (or cold) email account while generating positive engagement signals. The goal is to establish a sender reputation that tells Gmail, Outlook, and other email service providers: this is a legitimate sender whose messages people want to receive.
Every email address has a reputation score that ESPs use to decide where your messages land. A brand-new mailbox has no reputation—it's a blank slate, which ESPs treat with suspicion. A mailbox that suddenly sends hundreds of emails with no prior history looks like a spammer. Warmup solves this by simulating the kind of organic email behavior that legitimate senders exhibit: consistent volume, back-and-forth conversations, and high engagement rates.
What Happens Without Warmup
Skip warmup and you'll see the consequences fast:
- Spam folder placement: ESPs flag your messages as suspicious and route them to spam or promotions tabs
- Hard bounces and blocks: High-volume sending from a cold domain triggers rate limits or outright blocks from major ESPs
- Domain blacklisting: Enough spam complaints and your entire domain gets blacklisted, affecting all mailboxes on that domain
- Wasted leads: Every email that lands in spam is a prospect you'll never reach—and you may not even know it's happening
For cold outreach teams, warmup isn't optional. It's infrastructure. You wouldn't launch a website without SSL; you shouldn't launch a campaign without warmup. If you're evaluating deliverability tools for cold outreach, warmup capability should be at the top of your checklist.
How Instantly's Email Warmup Works
Instantly operates a warmup network made up of thousands of real mailboxes belonging to other Instantly users. When you enable warmup on your account, your mailbox joins this network and begins exchanging emails with other mailboxes in it.
Here's what happens behind the scenes:
Automated Sending
Instantly sends emails from your mailbox to other mailboxes in the warmup pool. Volume starts low (a few emails per day) and gradually increases over days and weeks.
Positive Engagement
The receiving mailboxes open your emails, reply to them, and mark them as important. Some are moved out of spam back to the inbox. These engagement signals are exactly what ESPs look for when evaluating sender legitimacy.
Reputation Building
Over time, ESPs observe a pattern: this mailbox sends emails, people open them, people reply. The sender reputation score improves and your messages start consistently landing in the primary inbox.
Ongoing Maintenance
Even after your reputation is established, warmup continues running alongside your campaigns to maintain positive engagement ratios and counterbalance any negative signals from cold outreach.
Instantly's warmup emails are not visible in your regular inbox. They're automatically tagged and filtered so they don't clutter your mailbox. You'll see warmup activity in the Instantly dashboard but it won't interfere with your real conversations.
Warmup Settings in Instantly
When you configure warmup in Instantly, you control several parameters:
| Setting | What It Controls | Recommended Value |
|---|---|---|
| Daily Warmup Limit | Maximum number of warmup emails sent per day | 30–40/day for new accounts |
| Ramp-Up Increment | How many additional emails to add per day during ramp-up | 1–2/day (gradual is better) |
| Reply Rate | Percentage of warmup emails that receive replies | 30–45% |
| Read Emulation | Whether receiving accounts open and read the email | Enabled |
The default settings work well for most users. The main thing to avoid is cranking up the daily limit too high too fast—ESPs can detect unnatural volume spikes even if engagement is positive.
How to Set Up Email Warmup in Instantly: Step-by-Step
Before you enable warmup, you need your DNS authentication in place. Without SPF, DKIM, and DMARC properly configured, warmup will be fighting an uphill battle. If you haven't set up your email authentication records yet, do that first.
Prerequisites
- A dedicated sending domain (not your primary company domain—use a secondary domain like getacme.com or tryacme.com)
- Mailboxes set up with Google Workspace or Microsoft 365 (2–3 mailboxes per domain is the standard recommendation)
- SPF, DKIM, and DMARC records configured and verified for your sending domain
- An active Instantly account with available warmup slots
Step-by-Step Setup
Connect Your Email Account
In Instantly, navigate to Email Accounts and click Add New. Connect your mailbox via SMTP/IMAP or use the direct Google Workspace / Microsoft 365 integration. Verify the connection is active.
Enable Warmup
Once your account is connected, toggle Warmup to enabled. Instantly will immediately begin enrolling your mailbox in the warmup network.
Configure Warmup Settings
Set your daily warmup limit to 30–40 emails per day with a ramp-up increment of 1–2 per day. Set reply rate to 30–45%. Enable read emulation. These are conservative settings that work for most cold outreach scenarios.
Verify DNS Records
Use Instantly's built-in deliverability checker or an external tool like MXToolbox to confirm that your SPF, DKIM, and DMARC records are passing. Fix any failures before proceeding.
Wait 2–3 Weeks Before Launching Campaigns
This is the part most teams rush. Your warmup needs at least 14–21 days of consistent activity before your sender reputation is strong enough for cold outreach. Monitor your warmup dashboard during this period—you should see inbox placement rates climbing steadily.
Start Campaigns at Low Volume
When you begin outreach, start with 10–20 cold emails per mailbox per day. Gradually increase over the next 1–2 weeks. Keep warmup running alongside your campaigns—never turn it off.
If you're managing multiple sending domains, stagger your warmup start dates by a few days. This avoids having all your mailboxes hit the same ramp-up milestones simultaneously, which can look unnatural to ESPs that track patterns across related domains.
Email Warmup Timeline: What to Expect
Warmup isn't instant. Here's a realistic timeline for what you should see during the process:
| Week | What's Happening | Expected Metrics |
|---|---|---|
| Week 1 | Initial warmup activity begins. Low volume (5–15 emails/day). ESPs start observing your sending patterns. | Inbox placement: 60–75%. Some emails may still land in spam. |
| Week 2 | Volume increases (15–30 emails/day). Positive engagement signals accumulate. Reputation starts forming. | Inbox placement: 75–90%. Spam placement decreasing. |
| Week 3 | Volume stabilizes near your daily limit. Consistent engagement pattern established. Ready for low-volume outreach. | Inbox placement: 90%+. Safe to begin campaigns. |
| Week 4+ | Warmup maintains reputation alongside active campaigns. Gradually increase outreach volume. | Inbox placement: 95%+ with proper campaign hygiene. |
Teams that rush warmup and start sending campaigns after 3–5 days almost always see deliverability problems within the first week of outreach. Two to three weeks of warmup before your first campaign isn't lost time—it's the foundation that makes everything after it work.
Email Warmup Best Practices for Cold Outreach
1. Never Turn Off Warmup
This is the most common mistake. Once your campaigns are running, keep warmup active. Cold outreach naturally generates some negative signals—low reply rates, occasional spam complaints, bounces. Warmup counterbalances these by maintaining a steady stream of positive engagement. The ratio of positive to negative signals is what determines your reputation.
2. Use Dedicated Sending Domains
Never send cold email from your primary company domain. If your main domain gets blacklisted, it affects everything—your transactional emails, your marketing emails, your team's day-to-day communication. Use secondary domains (variations of your brand name) exclusively for outreach. Most teams maintain 3–5 sending domains with 2–3 mailboxes each. For more on building a proper cold email infrastructure, see our automation tools guide.
3. Respect Daily Sending Limits
A common rule of thumb: keep your total daily sending volume (warmup + campaigns) under 50–60 emails per mailbox per day. Going higher than this increases the risk of ESP rate limits and spam folder placement, even with strong warmup metrics.
| Mailbox Age | Max Warmup/Day | Max Campaign/Day | Total Daily Limit |
|---|---|---|---|
| Week 1–2 | 20–30 | 0 (warmup only) | 20–30 |
| Week 3 | 30–40 | 10–15 | 40–55 |
| Week 4+ | 30–40 | 20–30 | 50–70 |
| Established (2+ months) | 20–30 | 25–35 | 45–65 |
4. Authenticate Everything
SPF, DKIM, and DMARC are non-negotiable. Google and Microsoft tightened enforcement throughout 2025 and into 2026. Without all three properly configured, your warmup is fighting against technical failures that no amount of positive engagement can overcome.
- SPF: Authorizes which servers can send on behalf of your domain
- DKIM: Adds a cryptographic signature to verify email authenticity
- DMARC: Tells ESPs what to do with emails that fail SPF or DKIM checks
5. Monitor Deliverability Metrics
Instantly's dashboard shows your warmup health score, inbox placement rate, and spam rate. Check these at least twice a week. If you see inbox placement dropping below 85%, pause your campaigns and investigate before it gets worse. Common culprits include: sending volume too high, poor email copy triggering spam filters, or DNS records that silently failed.
6. Write Emails That Don't Look Like Spam
Warmup handles the infrastructure side, but your campaign content matters too. ESPs increasingly use content analysis alongside reputation signals. Avoid known spam triggers:
- HTML-heavy emails with lots of images, links, or formatting
- All caps in subject lines
- Excessive exclamation marks or urgency language
- Link-heavy messages (keep to 1–2 links maximum)
- Generic, non-personalized content that looks mass-produced
The best cold emails read like they were written by a real person to one specific recipient. If you're looking to improve personalization at scale, AI email personalization tools can help you generate tailored messages without sacrificing deliverability.
Common Warmup Mistakes That Tank Deliverability
If you're seeing inbox placement below 70% after 3 weeks of warmup, or if your placement suddenly drops after launching campaigns, one of these mistakes is likely the cause.
Mistake 1: Starting Campaigns Too Early
The number one warmup mistake. Teams get impatient and start sending cold emails after 3–5 days. Your reputation hasn't had time to build. Start campaigns no earlier than 14 days after warmup begins, and ideally wait the full 21 days.
Mistake 2: Ramping Volume Too Aggressively
Going from 0 to 50 emails per day in a week is a red flag for ESPs. Gradual ramp-up—1 to 2 additional emails per day—mimics organic growth. It takes longer but the results are dramatically better.
Mistake 3: Ignoring Bounce Rates
High bounce rates (above 3–5%) destroy sender reputation faster than almost anything else. Verify your email lists before launching campaigns. A good deliverability stack includes email verification as a standard step before any send.
Mistake 4: Using One Mailbox for Everything
Sending all your volume through a single mailbox concentrates risk. If that mailbox gets flagged, your entire outreach stops. Distribute volume across multiple mailboxes and domains. Most teams use 2–3 mailboxes per domain across 3–5 domains.
Mistake 5: Turning Off Warmup After Launching Campaigns
Some users disable warmup once campaigns are running, thinking it's "done." Warmup needs to stay active permanently. The positive engagement it generates is what keeps your reputation balanced as cold outreach introduces negative signals.
Integrating Warmup Into Your Cold Outreach Strategy
Warmup isn't a standalone activity—it's one layer in a cold email infrastructure stack. Here's how it fits with the other pieces:
| Layer | Purpose | Tools |
|---|---|---|
| Domain Setup | Dedicated sending domains with DNS authentication | Google Workspace, Microsoft 365, Cloudflare |
| Email Warmup | Build and maintain sender reputation | Instantly warmup, Smartlead warmup |
| List Building | Find and verify prospect email addresses | Apollo, Clay, Instantly B2B Lead Finder |
| Email Verification | Remove invalid addresses before sending | ZeroBounce, NeverBounce, MillionVerifier |
| Personalization | Write relevant, tailored messages at scale | AI personalization tools, Octave |
| Sending | Automated campaign execution with rotation | Instantly, Smartlead, Lemlist |
| Monitoring | Track deliverability, open rates, reply rates | Instantly analytics, GlockApps, Mail-Tester |
When each layer is working, the results compound. Warmup establishes the reputation. Verification prevents bounces from damaging it. Personalization drives real engagement that reinforces it. Automation tools manage the orchestration.
The weakest link in this chain determines your ceiling. A perfect warmup won't save you if you're sending to unverified lists. Verified lists won't help if your messages are generic templates that get ignored. And none of it matters if your DNS authentication is misconfigured.
Where AI Personalization Fits In
One trend we've seen accelerate in 2026 is the connection between email personalization and deliverability. ESPs are increasingly sophisticated at detecting mass-produced content. When every email in a campaign has the same structure with one swapped variable, that pattern itself becomes a signal.
Tools like Octave help solve this by generating genuinely varied, context-aware email sequences. Rather than templated personalization (swapping in a first name and company), context-driven personalization produces structurally different messages for different prospects based on their role, company, and situation. This variation is better for engagement rates and better for deliverability, since ESPs see unique content rather than a pattern. You can explore more approaches in our roundup of AI email personalization tools.
Instantly Warmup vs. Other Warmup Solutions
Instantly isn't the only warmup option. Here's how it compares to the main alternatives:
| Feature | Instantly | Smartlead | Lemwarm (Lemlist) | Warmbox |
|---|---|---|---|---|
| Warmup included in plan | Yes (all plans) | Yes (all plans) | Yes (with Lemlist subscription) | Standalone product |
| Network size | Large (200K+ mailboxes) | Large | Moderate | Moderate |
| Unlimited mailboxes | Yes (Growth plan+) | Yes | No (per-seat) | Per-inbox pricing |
| Deliverability dashboard | Built-in | SmartDelivery | Basic | Detailed reports |
| Best for | Teams scaling cold email with many mailboxes | Agencies needing multi-channel + warmup | Teams already using Lemlist | Standalone warmup needs |
For most cold outreach teams, using warmup within your sending platform (Instantly or Smartlead) is the simplest approach. It keeps your warmup and campaign activity on the same infrastructure, making it easier to manage daily limits and monitor deliverability in one place.
For a deeper comparison, see our guide to deliverability tools for cold outreach.
Frequently Asked Questions
A minimum of 2 weeks, ideally 3 weeks. This gives ESPs enough data to form a positive reputation for your mailbox. Starting campaigns earlier significantly increases the risk of spam folder placement.
Yes, but it takes longer. A mailbox with a damaged reputation needs 4–6 weeks of warmup to recover, and some never fully recover. In many cases, it's faster and more reliable to set up a new mailbox on a new domain and start fresh.
Absolutely. Never turn off warmup. It maintains the positive engagement ratio that keeps your sender reputation healthy. Cold outreach naturally generates some negative signals, and warmup is what counterbalances them.
30–40 warmup emails per day is the standard recommendation. Combined with your campaign volume, keep the total under 50–60 per mailbox per day. Higher volumes increase risk without proportional benefit.
Warmup emails are separate from your campaign sending limits in Instantly. However, your ESP (Google Workspace or Microsoft 365) counts all outgoing emails toward their daily sending limits, so factor in both warmup and campaign volume when planning.
Most teams use 3–5 sending domains with 2–3 mailboxes per domain. This gives you 6–15 mailboxes to rotate across, distributing volume and risk. At 20–30 campaign emails per mailbox per day, that's 120–450 cold emails daily. Scale up by adding more domains, not by pushing higher volume per mailbox.
Warmup builds your sender reputation by generating positive engagement signals. Email rotation distributes your campaign sends across multiple mailboxes so no single mailbox takes too much volume. They work together: warmup keeps each mailbox healthy, and rotation spreads the load. Instantly handles both natively—you can learn more about rotation in our cold email automation guide.
Good warmup metrics confirm your infrastructure is healthy. Low open rates on campaigns usually point to content or targeting issues: subject lines that don't resonate, emails landing in promotions (not spam), or you're reaching the wrong audience. Check your campaign-level analytics separately. Also consider whether your email copy could benefit from better personalization.
Making Warmup Work for Your Outreach
Email warmup in Instantly is straightforward to set up but easy to get wrong if you rush the process. The teams that see consistently high deliverability follow the same playbook: authenticate their DNS, warm up for 2–3 weeks before any outreach, start campaigns at low volume, and keep warmup running permanently.
The investment is patience, and the payoff is substantial. A properly warmed infrastructure means your carefully crafted cold emails actually reach the inbox. Without it, even the best messaging and targeting are wasted.
As you scale your outreach, remember that warmup is one piece of a larger system. Domain management, list verification, campaign automation, and personalization all play a role. Get the fundamentals right and the results follow.
If you're building outbound at scale and want to ensure your messaging is as strong as your infrastructure, Octave helps teams generate context-aware email sequences that drive engagement and reinforce deliverability. Your warmup keeps you out of spam—your messaging is what gets replies.