1. List fatigue

A gradual decline in response rates over time—commonly referred to as list fatigue—is a natural occurrence in email marketing. It stems from a mix of factors: the growing volume of emails every recipient has to manage, the increasing frequency of sends, and the ongoing challenge for marketers to maintain consistently relevant content. On top of that, you have issues like inactive or abandoned email addresses, more aggressive spam filters, and the diminishing performance of older contacts compared to newly acquired ones. New addresses almost always yield higher response rates than those that have been on your list for a while.

The key question is: How can senders slow this decline in a cost-effective way? One of the most effective approaches is deeper personalisation—not just in content, but also in the sender name and timing of the emails. Ensuring every message feels timely and tailored makes a big difference in keeping engagement levels up, even as your list matures.

2. The long tail of conversions

Most email campaign responses happen quickly—50% within the first 24 hours, and 66% within 48 hours. But here’s the surprise: a significant chunk of conversions—sometimes over one-third—happen a week or more after the send, otherwise known as the long tail of email.

Even more interesting: those late conversions often have higher average order values. That means the later a recipient engages, the more valuable their action tends to be.

Key takeaway: Email is a dual powerhouse—it supports both immediate and delayed conversions. Marketers should adjust their ROI models and attribution logic accordingly.

3. Nichtreagierer or non-responders

The optimal handling of long-term Nichtreagierer (better known as non-responders) is one of the more controversial topics in email marketing. Opinions range from “consistently remove them” to “occasionally target them with reactivation campaigns,” all the way to “continue sending to them regularly.” Each approach has valid arguments. For example, Zustellrisiken—such as recycled spam traps and engagement-based spam filters—can increase when you continue sending to inactive addresses.

On the other hand, removing these contacts altogether carries a revenue risk, as you may lose the subtle benefits of subliminal marketing. After all, even if someone doesn’t open your email, the presence of your brand in their inbox—through sender name and subject line—can still have a measurable impact. Studies show that regularly contacted non-responders often generate more revenue over time than those who are completely dropped from communication.

The best way to determine the right strategy for your audience is through A/B split testing. Try different approaches, compare the outcomes, and base your decision on data rather than assumptions.

4. Terminal diversity: devices & conversion drop-offs

Due to the megatrend of increased mobile device usage, emails are now opened much faster after being sent than they were 10 years ago. That’s a positive development—and it clearly underlines the importance of taking mobile optimisation of email templates seriously. Most B2C senders already passed the milestone of “mobile email now,” meaning more than 50% of their emails are being opened on smartphones.

However, the bigger challenge for email marketers lies beyond the open rate: click-to-open rates and conversion rates on smartphones are significantly lower than on laptops or desktops. (Tablet behaviour, by contrast, is closer to laptops.)

And this challenge is likely to grow. More and more emails are now being delivered to—and in some cases read on—devices like smartwatches, data glasses, or even via voice assistants like Alexa. In the case of voice interaction, it’s not even clear how opens or clicks can be cleanly measured, adding to the complexity.

The impact of the device (“terminal”) on delayed conversions hasn’t yet been studied in depth. But it’s likely that a click in an email—especially on mobile—is just the first step in a longer conversion process. Many users may be bookmarking products, saving them to shopping lists, or otherwise postponing the purchase for later. This highlights the need to think beyond the initial interaction.

To help mitigate the erosion in conversion rates on mobile, there are several best practices that should be implemented:

• All newsletter links must remain functional for several weeks after sending.

• The entire post-click journey, from landing page to payment, must be consistently optimised for mobile users.

• Test across different mobile platforms and screen sizes to identify any friction points that could derail a purchase.

As device variety continues to grow, marketers must adapt. Emails may be read—or even heard—via language assistants like Alexa. And once again, it remains unclear how interaction metrics like opens or clicks can be reliably captured in these cases.

In light of this, email marketers should take steps to ensure consistency and accessibility across all stages of the funnel—especially in mobile contexts. By doing so, they can minimise the losses caused by device-related friction and improve both short-term and long-tail campaign performance.

5. Unsubscribe rates

Unsubscribes are often seen as a minor issue in email marketing. After all, the numbers are typically low—usually under 0.2%—so they don’t raise alarms in reporting dashboards. But this is a dangerous oversight.

Here’s why: even small unsubscribe rates can have massive cumulative effects. And it’s not easy to replace those lost contacts.

Acquiring high-quality email addresses has become significantly harder than it was a decade ago. Increased data protection regulations—like the GDPR—have led to more friction in sign-ups, stricter opt-in requirements, and in many cases, the loss of valuable contacts during compliance transitions. As a result, many senders have seen their lists stagnate or even shrink over time.

This makes reducing unsubscribe rates more important than ever. What can help reduce unsubscribe rates?

• Review opt-out reasons and analyse when and why people unsubscribe

• Use preference centres for better topic alignment

• Limit ad pressure and offer relevant incentives

• Personalise sender names and content

• Optimise your content for mobile consumption

Even a tiny reduction in unsubscribe rate can have huge long-term value.

6. Deliverability and the blind spots

When people think about email marketing performance, they usually jump straight to opens, clicks, and conversions. But beneath the surface lies something even more fundamental: deliverability. If your emails aren’t reaching the inbox in the first place, all other metrics become irrelevant.

Deliverability issues may start small, but over time they can snowball into serious revenue losses—especially if left unnoticed. Many senders aren’t even aware that they have a deliverability problem, particularly when using third-party tools or shared IPs where inbox placement isn’t always transparent. Worse, most dashboards don’t show how many emails actually landed in the inbox versus the spam folder, making this a critical blind spot.

That’s why it’s important to monitor and improve not only your sender reputation and bounce management, but also how your mailing infrastructure handles long-term list hygiene and feedback loops.

Some common long-term traps to keep an eye on:

• Using email platforms that don’t participate in certified programs like the Certified Senders Alliance

• “Outbouncer” effects: valid addresses being blocked after soft bounces, silently shrinking your list

• Blind spots in analytics: many email tools don’t report which addresses were silently dropped

How to address it:

• Regular A/B testing with different ESPs

• Use email validation tools to recover wrongly blocked addresses, like the Maileon AddressChecker.

• Keep an eye on inbox placement—not just delivery rates

7. Email address quality

There’s a widespread belief that B2B addresses outperform B2C (freemail) addresses—but that’s not always true.

Yes, B2B addresses often have higher open rates, but they’re also more prone to churn when employees switch jobs. In contrast, personal Gmail or Hotmail addresses can remain active for years, potentially delivering more lifetime engagement.

Also noteworthy: Gmail has now overtaken other providers to become the most-used email client globally. Its increasing dominance—especially among SMBs—means you must test your templates and optimize for Gmail rendering.

Bonus Tip: Embrace interactivity. Google’s AMP for Email lets you create in-email experiences (like forms and image galleries) without sending users to a landing page. This can significantly boost engagement—if your ESP supports it.

Final Thoughts

A healthy email program isn’t just about what happens in the first 24 hours. It’s about long-term impact, subtle metrics, and slow-building results. Pay attention to the deeper dynamics—like list fatigue, delayed conversions, device usage, and unsubscribe trends—and you’ll uncover powerful levers for long-term growth.