> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sendkit.dev/llms.txt
> Use this file to discover all available pages before exploring further.
# Domain Warmup
> Understand why new domains need to be warmed up and how the process works.
## 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.
## 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:
| Behavior | Provider reaction |
| ------------------------------------- | ---------------------------------------------- |
| Gradual volume increase over days | "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" |
## How SendKit warmup works
SendKit has a **built-in, automatic warmup system**. When you verify a new domain and send your first email, the 7-day warmup period begins.
SendKit automatically detects your domain's age and applies the appropriate warmup schedule. Younger domains get a more conservative schedule because they have no established reputation anywhere — not just on SendKit.
### Young domains (less than 30 days old)
| Day | Daily limit |
| --- | ------------ |
| 1 | 50 emails |
| 2 | 100 emails |
| 3 | 250 emails |
| 4 | 500 emails |
| 5 | 1,000 emails |
| 6 | 1,500 emails |
| 7 | 2,000 emails |
| 8+ | No limit |
### Established domains (30+ days old)
| Day | Daily limit |
| --- | ------------ |
| 1 | 200 emails |
| 2 | 500 emails |
| 3 | 1,000 emails |
| 4 | 2,000 emails |
| 5 | 3,500 emails |
| 6 | 5,000 emails |
| 7 | 7,500 emails |
| 8+ | 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.
### 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:
```json theme={null}
{
"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.
## Best practices during warmup
### 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.
### 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.
### 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.
### 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.
## Free plan daily limit
In addition to the warmup schedule, **Free plan accounts are limited to 100 emails per day**, even after warmup completes. This limit resets at midnight (UTC). Upgrading to a paid plan removes the daily limit.
## Warmup and campaigns
If you're planning a campaign during the warmup period, keep in mind that SendKit respects both the daily warmup limit and the daily plan limit. If either 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 and daily limit to avoid partial sends.
## 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 7 days.
Yes. Each verified domain has an independent warmup period. Adding a new domain starts a fresh 7-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.
SendKit automatically detects your domain's registration date. Domains registered less than 30 days ago use the young schedule, which has lower daily limits to protect your reputation during the critical early period. If we can't determine your domain's age, it defaults to the young schedule.