The critical importance of IP warming: a case study in email deliverability

Email deliverability is the silent foundation of successful digital marketing campaigns. Yet, one crucial aspect—IP warming—is often misunderstood or undervalued, especially by those new to the field. This post explores why IP warming is not just a best practice but an absolute necessity based on real marketing scenarios.

Before diving into our case study, let's clarify what IP warming actually is: the process of gradually increasing the volume of emails sent from a new IP address to establish sender reputation with Internet Service Providers (ISPs). This methodical approach helps demonstrate that you're a legitimate sender rather than a spammer.

Several years ago, a major enterprise with a substantial B2B marketing presence decided to switch Email Service Providers. Their US segment alone contained over 200,000 recipients, with historical deliverability rates in the upper 90th percentile—impressive by any standard.

During the transition, a new marketing team member challenged conventional wisdom, questioning whether IP warming was actually necessary or just an industry myth. Despite warnings from more experienced colleagues, leadership approved sending to the entire list at once.

The results were immediate and devastating. Deliverability collapsed to below 18.5%, causing significant revenue impact and drawing unwanted executive attention.

Following this incident, the company implemented a rigorous approach:

• Extremely conservative sending volumes to begin
• Daily monitoring of deliverability metrics and sender scores
• Detailed reporting to leadership
• Gradual scaling based on performance data

While the situation eventually stabilized, the company had already lost valuable marketing momentum and potentially damaged their sender reputation with both ESPs and recipients.

In a similar but separate incident, the same organization later transitioned their B2C email program (with an even larger recipient base) to a new ESP. Even with the previous lesson in mind, their initial send volumes were still too aggressive, causing another deliverability dip. They quickly course-corrected, adjusting subsequent sends and gradually rebuilding their sending reputation.

The challenges didn't end with proper warming. Other deliverability incidents highlighted the interconnected nature of email best practices:

• Third-party data purchases containing spam traps triggered a complete sending freeze for multiple weeks
• Data quality issues prompted intervention from the ESP itself when they detected unusual deliverability patterns

• Respect the process: IP warming isn't optional—it's fundamental infrastructure work
• Plan accordingly: Factor warming periods into project timelines when changing ESPs
• Start with engagement: Begin with your most engaged subscribers
• Data hygiene matters: Even perfectly warmed IPs can be compromised by poor list quality
• Monitor actively: Watch deliverability metrics closely during the entire warming period
• Be patient: Building reputation takes time—rushing costs more than waiting

The financial impact of poor deliverability is substantial but often hidden. Undelivered emails represent lost revenue opportunities, while damaged sender reputation affects future campaigns long after the initial incident.

For marketers planning ESP transitions or launching new email programs, remember that IP warming isn't just a technical recommendation—it's business protection. The time invested in proper warming will pay dividends through consistent deliverability and predictable marketing performance.

Have you experienced similar challenges with email deliverability? Share your thoughts in the comments below.