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What is domain warmup?

Domain warmup is the process of gradually increasing the volume of emails sent from a new domain over time. New domains have no sending history — email providers don’t know whether to trust them. Sending a large volume immediately triggers spam filters and can permanently damage your domain’s reputation. Warmup builds trust by showing email providers a pattern of legitimate, low-volume sending that gradually increases.

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Why warmup matters

Email providers use historical sending data to evaluate senders. A domain that has never sent email and suddenly starts sending thousands of messages looks suspicious — this is exactly what spammers do. Here’s how email providers react to new domains:
BehaviorProvider reaction
Gradual volume increase over weeks”This looks like a legitimate new sender”
Sudden high volume from day one”This looks like spam or a compromised domain”
Consistent daily volume”Predictable, trustworthy pattern”
Erratic volume (0, 0, 5000, 0, 10000)“Unpredictable, suspicious pattern”

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How SendKit warmup works

SendKit has a built-in, automatic warmup system. When you verify a new domain and send your first email, the 14-day warmup period begins.

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Warmup schedule

DayDaily limit
0–150 emails
2100 emails
3200 emails
4500 emails
5–61,000 emails
7–92,000 emails
10–135,000 emails
14+No limit
Daily limits reset at midnight (UTC).
The warmup clock starts when you send your first email from the domain, not when you verify it. You can verify your domain in advance without starting the warmup timer.

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What happens when you hit the limit

If you try to send more emails than your current daily limit allows, the API returns a 429 response: 
{
  "name": "warmup_limit",
  "message": "Domain warmup limit reached for example.com. Daily limit: 200 emails. Sent today: 200. Warmup completes on 2026-03-14.",
  "domain": "example.com",
  "daily_limit": 200,
  "emails_sent_today": 200,
  "warmup_day": 3,
  "warmup_ends_at": "2026-03-14T00:00:00Z"
}
Use the warmup_ends_at and daily_limit fields to plan your sending.

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Best practices during warmup

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Send to engaged recipients first

During the first few days, prioritize recipients who are most likely to open and interact with your emails:
  • New signups who just created an account
  • Users who recently used your product
  • Contacts who opted in directly
Positive engagement (opens, clicks, replies) during warmup helps build your reputation faster.

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Avoid cold or old lists

Do not send to purchased lists, scraped addresses, or contacts who haven’t engaged in months. High bounce rates and complaints during warmup can permanently harm your domain’s reputation before it’s even established.

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Send every day

Try to send emails every day during the warmup period. Consistent daily sending signals to email providers that you’re a legitimate, active sender. Gaps in sending don’t reset the warmup, but they reduce the strength of the reputation signal.

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Monitor metrics closely

Check your SendKit dashboard daily during warmup:
  • Bounce rate should stay under 5%
  • Complaint rate should stay under 0.1%
If any of these metrics spike, pause sending and investigate before continuing.

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Warmup and campaigns

If you’re planning a campaign during the warmup period, keep in mind that SendKit respects the daily warmup limit. If the limit is reached mid-campaign, the campaign is marked as partially sent — the remaining contacts won’t receive the email until you send the campaign again after the daily limit resets. Plan your campaign size around your current warmup day to avoid partial sends.

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Common questions

No. The warmup schedule is enforced automatically to protect your sending reputation. Skipping it risks having your emails flagged as spam from day one, which is much harder to recover from than waiting 14 days.
Yes. Each verified domain has an independent warmup period. Adding a new domain starts a fresh 14-day warmup for that domain only.
The warmup is based on calendar days since your first email, not sending days. If you skip a day, the schedule continues — your limits still increase as planned.
Yes. Batch sends count against your daily warmup limit. If a batch would exceed the limit, the entire batch is rejected.
Yes. Your reputation with another service is tied to their sending infrastructure (IPs). When you send through SendKit, the email comes from different infrastructure, so email providers need to learn to trust this new combination of your domain + SendKit’s servers.