Definition
Email cold warmup, also called IP or domain warmup, is the process of gradually increasing sending volume from a new domain or IP address to establish reputation with mailbox providers. New sending infrastructure has no reputation history, so mailbox providers treat it with suspicion. A sudden burst of volume from an unknown sender is almost guaranteed to trigger spam filtering or outright blocking.
The warmup schedule follows a structured volume ramp over 6 to 8 weeks. Week one and two involve sending to the most engaged subscriber segment only, typically those who have opened an email in the last 30 days, at volumes of 1,000 to 5,000 messages per day. Week three and four increase volume to moderately engaged subscribers, those who opened in the last 60 days, at 5,000 to 20,000 per day. Week five and six approach target volume by adding less engaged segments, reaching 80-100% of planned daily volume. Week seven and eight allow full volume sending, with ongoing monitoring.
Engagement-based warming differs from volume-based warming in its approach. Engagement-based warming prioritises sending to subscribers who are most likely to open and click, regardless of the total volume sent each day. The goal is to generate positive engagement signals that build reputation quickly. Volume-based warming prioritises reaching specific volume targets each day, with less emphasis on engagement quality. Most deliverability experts recommend engagement-based warming because mailbox providers increasingly weight engagement signals more heavily than volume in reputation calculations.
Best Practices
- Prepare your sending infrastructure before starting warmup: Configure SPF, DKIM, and DMARC authentication and verify they are passing. Set up a custom return-path domain. Configure feedback loops with major ISPs. Establish monitoring dashboards for bounce rates, complaint rates, and inbox placement before sending the first warmup message.
- Segment your most engaged subscribers for initial warmup sends: Identify the top 10-15% of your list by recent engagement, defined as subscribers who opened an email in the last 14-30 days. Send to this segment only during the first two weeks of warmup. Their positive engagement signals establish the reputation foundation for later volume increases.
- Monitor bounce rates, complaint rates, and inbox placement daily during warmup: Set alert thresholds: hard bounce rate below 2%, spam complaint rate below 0.1%, and inbox placement rate above 95%. If any metric exceeds its threshold, pause volume increases and send at the current level for an additional 3-5 days until metrics stabilise.
- Do not warm multiple IPs simultaneously from the same domain: Warming multiple IPs at the same time divides engagement signals across multiple reputations, slowing the warmup for each IP. Warm one IP at a time, moving to the next only when the current IP has reached full volume and stable delivery metrics.
- Extend the warmup schedule if metrics show instability: The 6-8 week schedule is a minimum for healthy lists. If your engagement metrics are lower than expected, if your list contains older data, or if you are sending to a new geographic region, extend each warmup phase by an additional week. A slow warmup that succeeds is far better than a fast warmup that damages reputation.
Related Glossary Terms
Abuse Complaint
An abuse complaint is a report from a recipient who marks an email as spam, which negatively affects sender reputation and deliverability.
AI Content Detection
AI content detection refers to the growing ability of email clients, spam filters and consumers to identify machine-generated email copy. It matters for deliverability, trust and engagement in an era of mass-produced AI marketing email.
Batch Sending
Batch sending is the practice of delivering a single email campaign to a large list in groups or waves rather than all at once.
Bounce Classification
Bounce classification uses SMTP codes (550, 551, 552, 553, 554, 450, 451, 452) and enhanced status codes to categorise permanent and transient delivery failures.
Bounce Management
Bounce management is the process of handling emails that are rejected by mailbox providers, including classification, removal of bad addresses and protection of sender reputation.
Complaint Rate
Complaint rate is the percentage of delivered emails that recipients mark as spam, a key indicator of sender reputation and list quality.
Frequently Asked Questions
Mailbox providers use reputation as a primary factor in inbox placement decisions. A new domain or IP has no reputation, meaning the provider has no data on whether the sender is legitimate or a spammer. Without reputation data, providers apply strict filtering that typically routes unknown sender traffic to spam. Warmup gradually builds reputation by generating positive engagement signals from known-good subscribers.
IP warmup builds reputation for a specific sending IP address, which is important for dedicated IP senders. Domain warmup builds reputation for the sending domain, which is used by mailbox providers even when sending from shared IPs. Both are necessary. ES Ps using shared IPs may require only domain warmup, while dedicated IP senders must warm both simultaneously.
Start with 1,000 to 5,000 emails per day, sent only to your most recently engaged subscribers. The exact starting volume depends on your total list size and ISP requirements. The key principle is to start small enough that any reputation damage from unexpected issues is contained, then increase gradually as positive metrics confirm the domain and IP are building reputation.
Sending volume that significantly exceeds the mailbox provider's expected ramp triggers spam filtering, causing most of your messages to land in spam. Severe over-sending can result in domain or IP blacklisting, requiring a much longer recovery period or abandonment of the sending infrastructure and starting the warmup process again from the beginning.
A domain with existing negative reputation, such as one that has been used for spam or has unresolved blacklist entries, typically cannot be warmed up successfully. The best approach is to abandon the damaged domain and start with a fresh domain that has no reputation history. If the domain must be used, the remediation process takes 3-6 months of strict sending practices and may require certification to restore reputation.