Proper domain warmup is crucial before sending out any email campaign. If you haven’t sent emails for a long time or are planning your very first send, your messages might end up in the spam folder. A sudden increase in email volume immediately raises red flags for receiving email services.
Domain warmup in email marketing is the process of gradually increasing the volume of emails sent from a new or previously inactive domain. The goal is to build a good sender reputation with internet service providers (ISPs) and email service providers (ESPs), which significantly increases the chances of your emails landing in the inbox.
Here’s your step-by-step guide to warming up your domain and significantly boosting your email deliverability.
Table of Contents
1. What a Domain Is and Why Its Reputation Matters for Email
A domain is a unique name or address for a website, such as nicesender.com.
Domain reputation indicates how trustworthy an organization’s emails are to email service providers and how often users complain about them.
There is no single system for calculating this rating. Each email provider collects its own statistics to determine a domain’s reputation, so the rating can vary.
2. Preparing for Your First Campaign
2.1. Cleaning Your Email List: What Is Validation and Why You Need It
Before your first email send—or after a break of more than 3 months—proper preparation of your subscriber list is crucial.
Next, mailing list validation may be necessary.
Validation is the process of checking your list and removing non-existent or inactive email addresses, unsubscribed users, and spam traps.
Validation improves the quality of your list, reduces the likelihood of landing in spam, boosts deliverability, saves money by only sending to active addresses, and protects your domain’s reputation.
You need to validate your mailing list when:
- You’ve taken a break from sending emails for more than 3 months, which could have led to contacts becoming inactive or turning into spam traps.
- Data was entered manually, which increases the chance of errors and typos.
- Contacts were added without double opt-in, increasing the likelihood of misspelled or incorrect addresses.
- You’ve previously sent emails through another service.
* Remember to upload your unsubscribe lists to NiceSender to avoid sending emails to these people, which prevents spam complaints and negative feedback.
Validating your email addresses is a critical step before sending a campaign. It ensures your messages are only sent to existing addresses, maximizes the chances of a successful delivery to the inbox, and minimizes the risk of harming your domain’s reputation.
For your first send, it’s best to target your most engaged and loyal segment. For example, those who recently subscribed, have opened emails in the past, or actively use your product or service.
2.2. Creating Your First Email
For an effective warm-up, create a first message that informs subscribers about your company and offers immediate, real value. This also serves as a reminder of who you are and why they signed up for your emails.
Pay attention not only to the email’s body but also to the sender details (name, subject line, preheader) and the footer. For a detailed guide on how to create an effective email that drives results, read our full blog post here.
2.3. Authenticating Your Domain
Domain authentication is an essential step that improves email deliverability and reduces the likelihood of messages being flagged as spam.
Authentication is a mandatory requirement, required by the sending policies of major providers like Google, Outlook, and Yahoo. Failure to comply can negatively affect your deliverability.
This is a one-time procedure that involves adding a special key to your DNS record. When an email service receives your message, it checks this key to confirm that the email was genuinely sent from your domain.
After authentication, always use an email address that belongs to the authenticated domain, for example: name@yourdomain.com.
In NiceSender, you can add a sender email in the Settings menu — Email senders list.
Find the full, step-by-step domain authentication guide here.
3. Gradual Email Sending: The Domain Warmup Process
For clarity, here’s a domain warmup schedule for a hypothetical list of 50,000 subscribers:
| Day | % of Total List | Number of Emails (Examples) | Recommendation |
|---|---|---|---|
| 1 | 10% | 5,000 | First test send, check key metrics. |
| 3 | 20% | 10,000 | Analyze metrics, continue sending. |
| 5 | 40% | 20,000 | Monitor complaint rate (target less than 0.5%). |
| 7 | 70% | 35,000 | Evaluate open rates and audience engagement. |
| 9 | 100% | 50,000 | Full send to the entire list if metrics are good. |
Important points:
- Adjust the volume to your specific list size, maintaining the percentage-based proportions.
- On the specified days, analyze deliverability metrics, complaint rates, and open rates.
- If you notice any signs of a decline in your metrics, slow down the increase in sending volume or maintain the current volume for a few more days.
Gradual email sending is a proven warmup strategy that helps improve deliverability and prevent messages from being marked as spam.
The time it takes to warm up a domain depends on the mailing list size and domain reputation. If your mailing list has fewer than 100 subscribers, domain warmup is generally not necessary.
If your domain already has a good reputation, you can begin your warmup with 5,000–10,000 recipients immediately. After 1–2 days, double the recipient count, provided your complaint rate remains below 0.5%.
You can check your domain’s reputation using specialized services like Sender Score, Barracuda, SpamHous, and others.
If your domain’s reputation is below average, it’s best to start with a smaller batch of emails after a long break and gradually increase the volume.
First, send to 10% of your total mailing list. Then, check the statistics for delivered and opened emails, and the number of spam complaints.
After 1–2 days, send a second email to the same recipients plus an additional 10% of your mailing list. If the metrics are positive, add another 30–40% of recipients after 1–2 days, and so on, until you reach your entire mailing list.
*Ideally, you should send a new email each time, adding 10%, 30%, or 40% of new addresses to the previous recipient list. However, if you plan to send the same email for warmup, segment your audience and only resend it to those who received but did not open the previous email.
This incremental approach allows you to send more emails without triggering spam filters, effectively establishing a positive domain reputation over time.
Domain warmup can take anywhere from a few days to several weeks, depending on factors such as mailing list size, website traffic, and domain reputation.
4. Monitoring Key Metrics
There are three key metrics that indicate the effectiveness of your email campaigns:
- Open Rate
This metric shows how many people opened your email. Optimal open rates typically range between 10% and 25%. A lower rate may suggest a weak subject line or insufficient personalization.
- Bounce Rate
This is the percentage of emails that couldn’t be delivered. A bounce rate over 5% can cause your emails to be flagged as spam or even lead to your domain being blocked.
- Complaint Rate (Spam Rate)
This is the percentage of messages reported as spam by users. According to Google’s latest sender guidelines, you must maintain a spam rate below 0.1%.
A rate of 0.3% or higher is a critical threshold. Reaching this level will significantly degrade your inbox delivery and may lead to a complete block of your domain. If your spam rate stays at or above 0.3% for a period of time, you will become ineligible for mitigation until the rate drops below this threshold for 7 consecutive days.
To keep your rate optimal, monitor your metrics daily via Postmaster Tools and ensure your emails are sent only to engaged subscribers who have a clear option to unsubscribe.
To summarize, the goal of domain warmup is to build a good sender reputation, prevent emails from landing in spam, and improve deliverability. To achieve this:
1. Ensure the website on the domain you’re sending from is active and its content aligns with your emails.
2. Identify an engaged segment among your subscribers and clean your mailing list using validation.
3. Create an engaging welcome email that reminds subscribers who you are and the value they will receive.
4. Send your campaign:
- To your entire mailing list if it has fewer than 100 subscribers.
- To 5,000–10,000 recipients, and then double the count after 1–2 days if the complaint rate is below 0.5% and your website has a reputation of 50 or higher.
- To 10% of your mailing list, gradually increasing the volume if your domain’s reputation is below average.
5. Analyze your metrics (Bounce Rate, Complaint Rate, and Open Rate) to adjust your sending strategy as needed.
These useful tips will help you warm up your domain before your first email campaign or after a long break, ensuring your emails reach the inbox and drive positive results.