The Invisible Deliverability Killers Marketers Ignore [2025]

Email deliverability is more than just avoiding spam filters; it’s about creating sustainable engagement while protecting your sender reputation.

Unfortunately, many marketers underestimate the impact of small yet critical factors that can quietly undermine their email campaigns.

In this post, we’ll dive into these hidden challenges, or “deliverability killers” if you like, and share actionable tips to overcome them.

1. Frequency Fatigue

Sending too many messages in a short period can overwhelm subscribers, leading to email fatigue, increased unsubscribes, and complaints.

Example: A retail brand sent daily “flash sale” emails during the holiday season. While open rates initially spiked, complaints and unsubscribes increased by 30%, hurting their sender reputation.

Solution: Set frequency caps to limit the number of emails sent each week, and let subscribers customize how often they receive updates.

2. Timing Matters

Delivering emails at the wrong time, such as during national holidays or major local events, can drastically reduce engagement.

Example: A SaaS company launched a webinar invite campaign targeting North America during Thanksgiving weekend. Open rates plummeted to 5% compared to their usual 25%, wasting resources and opportunities.

Solution: Research regional holidays and time zones before scheduling campaigns to find the best time to send your campaigns. Also, don’t forget to A/B test to identify optimal engagement windows.

3. Accessibility in Emails

Ignoring accessibility features like alt text, contrast ratios, or readable fonts alienates a significant portion of your audience, including those with visual impairments.

Example: A fashion eCommerce company used flashy graphics-heavy emails with no alt text. Subscribers using screen readers received blank descriptions, leading to a poor user experience and lower click-through rates.

Solution: Follow accessibility best practices by adding descriptive alt text for images, using high-contrast color schemes, and ensuring font sizes are legible on all devices.

4. Domain History

Using a domain with a questionable or unknown history can harm your email deliverability, even if your content is legitimate.

Example: A startup used a domain purchased at auction without checking its history. Unbeknownst to them, the domain had previously been flagged for spam activity. As a result, their initial campaigns landed in spam folders.

Solution: Always check the history of domains using tools like MXToolbox or DomainTools before using them for email campaigns.

5. Email Weight

Oversized emails can trigger spam filters or take too long to load, frustrating recipients.

Example: A travel company embedded high-resolution videos directly in their emails, resulting in a 6MB file size. Their delivery rates dropped as ISPs flagged the emails for being too heavy.

Solution: Keep email sizes under 100KB by optimizing images and linking to external content like videos or PDFs instead of embedding them.

Slaying the Deliverability Killers

Improving email deliverability isn’t just about fixing technical issues—it’s about creating a seamless experience for your subscribers.

By addressing pitfalls like frequency fatigue, poor timing, or inaccessible designs, you ensure your emails are effective, engaging, and well-received. Thoughtful strategies lead to stronger connections and better results—your subscribers will thank you.