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When you start sending from a new domain or IP address, mailbox providers have no history to judge you by. Warm-up is the process of gradually increasing your sending volume so providers can observe positive engagement signals and build trust in your sender identity. Skipping or rushing this process is one of the most common causes of long-term deliverability problems.

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Why Warm-Up Matters

Mailbox providers like Gmail, Outlook, and Yahoo are designed to protect their users from spam. When they see a new domain suddenly sending thousands of emails, they treat it as suspicious — the same pattern spammers use with disposable domains. A proper warm-up demonstrates that:
  • Real recipients want your emails (they open, click, and reply)
  • You maintain a clean list (low bounce rates)
  • You respect recipient preferences (low complaint rates)
  • Your sending patterns are predictable and consistent
Sending high volumes immediately from a new domain can result in blocks that take weeks or months to lift. Some providers may permanently flag your domain, making recovery extremely difficult.

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Domain Warm-Up Schedule

This schedule applies when you add a new sending domain to Lettr. Adjust the numbers based on your total list size — the key principle is gradual, consistent growth.

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Low-Volume Senders (under 10,000 emails/month)

WeekDaily VolumeTarget AudienceKey Metrics to Watch
150–100Most engaged recipients (opened/clicked in last 7 days)Bounce rate, complaints
2200–500Engaged in last 30 daysBounce rate, complaints, deferrals
3500–1,000Engaged in last 90 daysAll metrics, inbox placement
4+Scale to targetFull active listAll metrics

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High-Volume Senders (over 10,000 emails/month)

WeekDaily VolumeTarget AudienceKey Metrics to Watch
1100–200Most engaged recipients (opened/clicked in last 7 days)Bounce rate, complaints
2500–1,000Engaged in last 14 daysBounce rate, complaints, deferrals
32,000–5,000Engaged in last 30 daysAll metrics, inbox placement
45,000–10,000Engaged in last 60 daysAll metrics
510,000–25,000Engaged in last 90 daysAll metrics
6+Scale toward targetFull active list (excluding unengaged)All metrics
These are guidelines, not rigid rules. The right pace depends on your engagement metrics. If bounce rates stay below 2% and complaint rates below 0.1%, you can accelerate. If either metric spikes, slow down immediately.

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Warm-Up Principles

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Send to Engaged Recipients First

Start with recipients who have recently interacted with your emails — opens, clicks, or replies in the last 7–14 days. These recipients are most likely to engage again, which sends strong positive signals to mailbox providers. 
// Example: Query engaged recipients for warm-up phases
const warmupRecipients = await db.recipients.find({
  lastEngagedAt: {
    $gte: new Date(Date.now() - 7 * 24 * 60 * 60 * 1000) // Last 7 days
  },
  status: 'active',
  suppressionStatus: null
}).limit(100); // Week 1 daily cap

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Send Consistently Every Day

Sending every day during warm-up is more important than sending large volumes. Sporadic bursts followed by silence look suspicious. A steady 100 emails per day is better than 700 emails once a week.

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Authenticate Before You Start

All DNS records must be verified and passing before you send your first warm-up email. This is non-negotiable.
1

Add your sending domain in Lettr

Navigate to Domains → Add Domain and enter your sending subdomain (e.g., mail.example.com).
2

Configure DNS records

Add the CNAME and DKIM records shown in your Lettr dashboard to your DNS provider.
3

Set up DMARC

Publish a DMARC record for your domain. Start with p=none during warm-up so you can monitor without risking rejections.
4

Verify all records pass

Wait for verification to complete in your dashboard. All records should show a green checkmark before you send.

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Monitor After Every Send

Check your metrics in the Lettr dashboard after each warm-up send. Set up webhook endpoints for real-time monitoring: 
app.post('/webhooks/lettr', (req, res) => {
  const event = req.body;

  switch (event.type) {
    case 'email.bounced':
      logWarmupMetric('bounce', event.data);
      break;
    case 'email.complained':
      logWarmupMetric('complaint', event.data);
      // Immediately suppress this recipient
      suppressRecipient(event.data.to, 'spam_complaint');
      break;
    case 'email.deferred':
      logWarmupMetric('deferral', event.data);
      break;
    case 'email.delivered':
      logWarmupMetric('delivered', event.data);
      break;
  }

  res.sendStatus(200);
});

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Stop Signals

Pause your warm-up and investigate immediately if you observe any of these:
SignalThresholdAction
Bounce rate> 5% on any single sendStop sending. Clean your list before resuming.
Complaint rate> 0.3%Stop sending. Review content and targeting.
Deferrals spikeSudden increase in email.deferred eventsReduce volume by 50%. Resume previous volume after deferrals normalize.
Block messages421 or 550 responses mentioning reputationStop sending to that provider. Wait 24–48 hours before retrying at reduced volume.
Do not ignore deferrals. A deferral means the receiving server accepted the connection but delayed delivery — it’s a warning that the provider is becoming cautious about your domain. Continuing at the same volume will likely escalate to blocks.

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Warm-Up by Provider

Different mailbox providers have different thresholds and behaviors during warm-up.

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Gmail

Gmail relies heavily on domain reputation and engagement signals. During warm-up:
  • Keep daily volume to Gmail addresses especially conservative (start with 20–50)
  • Gmail is the most sensitive to complaint rates — stay well below 0.1%
  • Use Google Postmaster Tools to monitor your domain reputation directly

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Outlook / Microsoft 365

Microsoft uses SmartScreen filtering and has its own reputation system:
  • Outlook tends to be more forgiving during initial warm-up than Gmail
  • Register with SNDS (Smart Network Data Services) to monitor your reputation
  • Outlook may silently route emails to Junk without bouncing — monitor open rates closely

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Yahoo

Yahoo was an early adopter of domain-based reputation:
  • Yahoo provides feedback loops (FBL) — ensure Lettr is processing these
  • Yahoo tends to defer rather than block during warm-up, so watch for deferral patterns

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Warming Up a Damaged Domain

If you’re recovering from reputation damage rather than starting fresh, the warm-up process is similar but stricter.
1

Reduce volume to near zero

Cut sending to 10–20% of your pre-damage volume, or pause entirely for 48 hours.
2

Identify and fix the root cause

Check webhook logs for email.bounced, email.complained, and email.deferred events. Common causes: stale list, broken authentication, sudden volume spike.
3

Clean your list aggressively

Remove all hard bounces, all complainants, and anyone who hasn’t engaged in 6 months.
4

Restart warm-up at week 1 volumes

Follow the warm-up schedule above, starting from week 1 with your most engaged recipients only.
5

Monitor daily for 4–6 weeks

Reputation recovery is slower than initial warm-up. Expect 2–6 weeks of consistent good behavior before providers fully trust your domain again.

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Common Mistakes

The most common warm-up mistake. Even if your list is opt-in and legitimate, sending thousands of emails from a brand-new domain triggers spam filters. Always start small and scale gradually.
Gaps in sending volume (like skipping weekends) create an inconsistent pattern. If you can’t send every day, at least send smaller volumes on off-days to maintain continuity.
Warm-up isn’t just about volume — it’s about engagement quality. If your open rates are low even during warm-up, your content or targeting needs work before you scale up.
Sending to your full list sorted by signup date instead of engagement recency. Always prioritize recipients who have recently opened or clicked.
Sending emails without verified DKIM, SPF, and DMARC records during warm-up wastes every positive signal you generate. Authenticate first, then warm up.

Sending Reputation

Build and protect your sender reputation over time

List Hygiene

Maintain clean, engaged recipient lists

Sending Domains

Configure and verify your sending domains

Deliverability Best Practices

Maximize inbox placement across all providers