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What is domain warmup?

When you verify a new domain on SendKit, it doesn’t have an established sending reputation with email providers like Gmail, Outlook, or Yahoo. Sending a large volume of emails immediately from a new domain can trigger spam filters and damage your deliverability. Domain warmup is SendKit’s built-in mechanism that automatically and gradually increases your daily sending limit over a 14-day period. This gives email providers time to recognize your domain as a legitimate sender.
You don’t need to configure anything. Warmup is fully automatic and starts when you send your first email from a verified domain.

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How it works

  1. You verify your domain — DNS records are configured and validated.
  2. You send your first email — This starts the 14-day warmup clock.
  3. Daily limits increase automatically — Each day, SendKit allows more emails from your domain.
  4. Warmup completes on day 14 — Your domain has no sending limits after the warmup period.
The warmup clock starts on your first email sent, not when you verify the domain. This means you can verify your domain ahead of time without affecting the warmup schedule.

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Warmup schedule

The daily sending limits increase progressively over 14 days:
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).

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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 Too Many Requests response with details about your warmup status: 
{
  "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 warmup_ends_at to know when your domain will be fully warmed up, and daily_limit to plan your sending volume accordingly.

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Warmup and campaigns

When sending a campaign, SendKit respects the warmup limit of each domain. If the limit is reached mid-campaign, the remaining contacts won’t receive the email until the next day’s limit resets. Plan your campaign volume around your current warmup day to avoid interruptions.

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Tips for a successful warmup

1

Start with engaged recipients

During the first few days, send to recipients who are most likely to open and interact with your emails. Positive engagement signals help build your domain’s reputation faster.
2

Avoid cold lists

Don’t use purchased or scraped email lists during warmup. High bounce rates and spam complaints during this period can permanently harm your domain’s reputation.
3

Monitor your metrics

Keep an eye on your bounce rate and complaint rate in the SendKit dashboard. If either spikes, pause sending and investigate before continuing.
4

Be consistent

Try to send emails every day during the warmup period. Consistent sending patterns signal to email providers that you’re a legitimate sender.

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FAQ

No. The warmup schedule is enforced automatically to protect your sending reputation. Skipping it would risk your emails being flagged as spam.
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.