What are the best domain warm-up strategies for cold outreach and how to avoid spam filters?
Michael Ko
Co-founder & CEO, Suped
Published 21 Jun 2025
Updated 16 Aug 2025
7 min read
When venturing into cold outreach, one of the most critical steps to ensure your emails actually reach the inbox, rather than the dreaded spam folder, is proper domain warm-up. This process is about building a positive sender reputation with mailbox providers. Just like you wouldn't trust a new, unverified person right away, email service providers (ESPs) are wary of new domains or domains that suddenly start sending high volumes of mail. Without a structured warm-up strategy, your well-crafted cold emails are likely to be flagged, blocked, or sent straight to spam, severely hindering your outreach efforts and potentially damaging your domain's long-term deliverability.
The goal of warming up is to establish a history of legitimate sending activity. This tells ESPs like Gmail and Outlook that your domain is a trustworthy source of email. It is a foundational step, especially for those embarking on cold email campaigns, where the risk of being perceived as spam is inherently higher. A well-executed warm-up protects your sender reputation and ensures your messages have the best possible chance of reaching their intended recipients' inboxes.
Why domain warm-up is essential
Domain warm-up is crucial because mailbox providers analyze your sending behavior to determine your sender reputation. A new domain has no history, so it's treated with extreme caution. Suddenly sending a large volume of emails from a cold domain will immediately trigger spam filters, leading to low deliverability and potential blacklisting (or blocklisting).
Without proper warm-up, you risk hitting spam traps, getting high bounce rates, and accumulating spam complaints. These negative signals quickly degrade your domain's reputation, making it nearly impossible to reach inboxes in the future. Once your domain is flagged, recovering your sender reputation can be a lengthy and challenging process. I often see people struggling to fix spam issues after previous cold outreach.
For cold outreach, it is also a best practice to use a separate subdomain or even a completely different domain from your primary brand domain. This strategy isolates your cold outreach activity, protecting your main domain's reputation from any potential negative impact. This is a common piece of advice in the industry for cold email deliverability.
Key strategies for effective domain warm-up
Effective domain warm-up involves a methodical approach to gradually build trust. The core idea is to simulate natural email sending behavior that ESPs recognize as legitimate. This means starting with low volumes and slowly increasing them over time, ensuring positive engagement along the way.
I recommend beginning with a very small number of highly personalized emails to engaged contacts or even internal team members. The initial focus is on getting opens, replies, and ideally, having recipients move your emails out of the spam folder if they land there. This positive interaction signals to ESPs that your mail is wanted.
Consistency is key. You should gradually increase your sending volume each day or week, following a ramp-up schedule. It is also important to diversify your recipient list across different mailbox providers, targeting mailbox providers during an email warmup based on your target audience. This helps build a reputation with each provider individually. For example, a sample warm-up schedule for cold outreach might look like this:
Day
Emails Sent
1-3
10-20
4-7
20-50
8-14
50-100
15+
100+ (Gradual increase)
Technical setup for optimal deliverability
A robust technical setup is non-negotiable for email deliverability. This includes correctly configuring SPF, DKIM, and DMARC records for your domain. These authentication protocols verify that your emails are legitimate and prevent spoofing, which is a major factor in emails landing in spam.
For SPF, ensure all your sending IPs are listed and avoid exceeding the 10-lookup limit. For DKIM, ensure your selector records are published correctly. DMARC, while not strictly required for initial warm-up, provides valuable feedback on your authentication status and should be implemented at p=none initially to gather reports without impacting delivery. You can check your email deliverability with a tester.
Here's how these records typically look:
SPF record exampledns
v=spf1 include:_spf.example.com ~all
DKIM record exampledns
s1._domainkey IN TXT "v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDnQ3K0F4..."
DMARC record example (p=none)dns
_dmarc.yourdomain.com IN TXT "v=DMARC1; p=none; rua=mailto:dmarc_reports@yourdomain.com; ruf=mailto:dmarc_forensic@yourdomain.com;"
Setting up these records correctly is the first technical hurdle for any new sending domain. For comprehensive guidance on configuring DMARC, you can use our free DMARC record generator tool.
Avoiding spam filters and maintaining reputation
Even with a warmed-up domain, spam filters are a constant challenge, especially with cold outreach. Content quality plays a huge role. Avoid spammy words, excessive capitalization, or too many exclamation points. Personalize your emails as much as possible to make them relevant to the recipient, fostering engagement and reducing the likelihood of them being marked as spam. A well-crafted, personalized email is less likely to trigger filters than a generic, salesy blast.
Maintaining a clean email list is equally important. Regularly verify your email addresses to minimize bounces and remove inactive contacts. High bounce rates signal a poorly managed list and negatively impact your sender reputation. Promptly honoring unsubscribe requests is also crucial, as complaints are heavily weighted by ESPs.
Proactive monitoring of your domain's reputation is vital. Use tools to check if your domain or IP is listed on any email blacklists (or blocklists). Being listed on a major blacklist can severely impact your deliverability. If you find your domain on a blocklist, act quickly to understand the cause and request delisting. Our blocklist monitoring service can help with this. You can also refer to our guide on how to warm up email domain effectively.
Positive actions for deliverability
Engagement focused content: Craft compelling subject lines and email bodies that encourage opens and replies. Encourage recipients to move your email to their primary inbox if it lands in spam.
Personalization: Tailor your messages to each recipient. Generic, bulk emails are more likely to be flagged as spam.
List hygiene: Regularly clean your email lists to remove invalid or inactive addresses. This reduces bounce rates and spam trap hits.
Views from the trenches
Best practices
Start with a very low sending volume and gradually increase it daily or weekly over several weeks.
Send emails to engaged contacts or internal addresses first to generate positive interactions.
Ensure all email authentication records (SPF, DKIM, DMARC) are correctly configured.
Monitor your sender reputation and blocklist status using available tools.
Segment your audience and personalize emails to increase engagement and reduce spam complaints.
Common pitfalls
Sending high volumes of emails from a brand new domain without any warm-up.
Ignoring email authentication, which makes your domain appear untrustworthy to ESPs.
Not regularly cleaning your email lists, leading to high bounce rates and spam trap hits.
Using generic, unpersonalized email content that can trigger spam filters.
Failing to monitor deliverability metrics, missing early signs of reputation issues.
Expert tips
Consider using a dedicated subdomain for cold outreach to protect your primary domain's reputation.
Actively encourage replies and engagement during the warm-up phase to signal good sender behavior.
If using an email warm-up tool, ensure it simulates realistic human interaction, rather than automated bulk sending.
Always prioritize email list quality over quantity to maintain high deliverability rates.
Understand that cold outreach, especially in bulk, carries inherent deliverability risks.
Expert view
Expert from Email Geeks says: If your emails are genuinely wanted by recipients, minimal warm-up is needed for these volumes. Many tools promising artificial interactions are often detected by mailbox providers, as this is considered shady behavior.
2022-06-08 - Email Geeks
Expert view
Expert from Email Geeks says: The most effective tool for warm-up is a solid strategy supported by ethical acquisition practices and prompt unsubscribe honoring. If your sending infrastructure is properly authenticated, your own well-executed plan is key.
2022-06-08 - Email Geeks
Maximizing cold outreach success
Effective domain warm-up is not just a technicality, it is a strategic imperative for successful cold outreach. It builds the necessary trust with mailbox providers, ensuring your legitimate messages reach the inbox instead of being caught by spam filters or landing on a blocklist. By gradually increasing sending volume, maintaining proper authentication, and focusing on quality content and list hygiene, you can cultivate a strong sender reputation.
Remember, the goal is to demonstrate consistent, desired email activity. This proactive approach not only optimizes your deliverability for current campaigns but also safeguards your domain's reputation for future outreach. Invest the time and effort into a meticulous warm-up strategy, and you will see a significant improvement in your cold email campaign performance.
Gmail and 
Outlook that your domain is a trustworthy source of email. It is a foundational step, especially for those embarking on cold email campaigns, where the risk of being perceived as spam is inherently higher. A well-executed warm-up protects your sender reputation and ensures your messages have the best possible chance of reaching their intended recipients' inboxes.
