One thing we see repeatedly in deliverability work is that problems rarely announce themselves. Most of the time, messages are still “sent successfully,” and ESP dashboards show no errors. But at the same time, inbox placement, engagement, and reputation decline, and campaigns underperform for no clear reason.
It is precisely in this gap between sending an email and its actual arrival in the “Inbox” that most companies fail. Modern mailbox providers evaluate far more than technical setup. They continuously assess sender identity, infrastructure consistency, recipient behavior patterns, and the quality of the messages being sent. It follows that deliverability is a dynamic metric that requires constant checking.
In this article, we break down the most serious email mistakes that can damage your deliverability, based on patterns observed through thousands of inbox placement tests.
Key Takeaways
- SPF, DKIM, and DMARC setup is not enough; consistent authentication monitoring helps spot issues and protect domains
- A healthy email list is one that consistently generates positive engagement signals
- Sending to large disengaged segments to hit volume numbers actively works against you
- Inbox placement depends on the combination of technical setup, sender behavior, recipient engagement, and message quality
- Email content is evaluated as a deliverability signal, not just a creative output
- Poorly structured HTML code, heavy images, and too many links can hurt email performance
- Pre-send testing helps identify deliverability risks before they affect large campaigns and revenue
- Stable sending patterns and continuous monitoring help maintain trust with mailbox providers
Top Email Mistakes You Should Fix to Improve Your Deliverability
1. Authentication that technically passes, but no longer reflects your sending setup.
Email authentication is essential for proving to Internet Service Providers (ISPs) and mailbox providers that you have the authority to send emails from your domain, and it serves as a fundamental practice for responsible email sending. Unfortunately, many businesses set up SPF, DKIM, and DMARC once and assume the job is done.
The problem is that email infrastructure changes constantly. A company may start with one sending platform, later add more for automation, and then introduce a product email platform. Each tool may have a legitimate reason to send emails, but nobody reviews the full authentication setup afterward. As a result, authentication can still “pass” while your domain setup no longer accurately represents who is sending emails on your behalf.
Another common issue is implementing DMARC but never moving beyond monitoring mode (p=none). This gives visibility but does not protect your domain from unauthorized use. However, moving to enforcement requires understanding legitimate sending sources first to avoid affecting valid emails.
What to fix:
- Review SPF, DKIM, and DMARC records regularly
- Monitor DMARC reports to identify unknown senders or authentication failures
- Remove outdated sending sources
- Check that your email streams are correctly aligned with your domain
Tools like DMARKOFF help businesses analyze DMARC data without extensive expertise, turning complex reports into clear authentication insights and action steps for fixing issues.
DMARKOFF is a next-generation DMARC monitoring platform that helps businesses protect their domains and email infrastructure. With clear authentication insights, user-friendly widgets, smart alerts, and multi-domain support, it helps you monitor multiple domains and collaborate more efficiently.
2. Poor list hygiene that slowly damages sender reputation.
List quality is one of the most underestimated deliverability factors because its impact is gradual rather than immediate. When businesses continue sending campaigns to people who have not engaged for months, mailbox providers see weaker engagement signals. Over time, this can reduce trust in the sender.
Common mistakes include keeping inactive subscribers indefinitely, ignoring bounced or invalid addresses, sending the same campaigns to engaged and disengaged users, and collecting contacts without a re-engagement strategy.
For example, your company collected contacts several years ago and may still have thousands of subscribers who have never interacted with recent campaigns. Sending every campaign to this group can lower overall engagement metrics, even when the active part of the audience remains healthy.
List hygiene is the process of maintaining a clean email database by removing invalid, inactive, or low-quality contacts to improve campaign performance and inbox placement.
What to fix:
- Clean your email list monthly (at least quarterly), removing invalid addresses
- Create inactive subscriber segments
- Run re-engagement campaigns before removing users
- Adjust sending frequency based on engagement
- Implement double opt-in for all new subscribers
- Never buy or scrape email lists to avoid spam traps
Remember, a smaller but active audience always performs better than a large list full of inactive recipients.
3. Ignoring engagement signals from your audience.
Mailbox providers pay close attention to how people interact with your emails. If users consistently ignore messages, delete them without reading, or mark them as spam, future emails may have a harder time reaching the inbox.
List hygiene determines who receives your emails. Engagement signals show mailbox providers what recipients do after receiving them.
A common mistake is focusing only on sending more campaigns instead of improving engagement quality.
What to fix:
- Monitor engagement trends, not only individual campaign results
- Stop sending regularly to inactive audiences
- Segment users based on behavior
- Create content that matches what subscribers expect from you
The goal is not maximum sending volume. The goal is to maintain positive signals with the people receiving your emails.
4. Treating email content as a design task instead of a deliverability factor.
Content affects more than clicks and conversions. It also influences how mailbox providers evaluate your messages. They evaluate message structure, including elements such as images, links, formatting patterns, and how your content aligns with recipient expectations.
Common content mistakes include:
- Sending repetitive promotional emails with little value beyond sales messaging
- Template patterns that look identical campaign after campaign, which some filters treat as bulk automation signals
- Using unclear subject lines that do not match the message
- Designing complicated HTML layouts
- Adding heavy images, too many links, or attachments
What to fix:
- Make the email content match subscriber expectations
- Check every link in your email, use recognizable domains, and avoid unnecessary redirects
- Avoid unnecessary attachments in marketing emails and use secure, trusted hosting options when sharing files
- Check the HTML code for issues and compatibility across different platforms
None of this means content is a primary deliverability lever. Reputation and technical foundation still matter more. But content is increasingly evaluated in context. A perfect email can still underperform if recipients consistently ignore it.
5. Skipping deliverability testing before sending campaigns.
If you think inbox placement is uniform across providers, that’s not true. The same email can reach the primary inbox at Microsoft, hit the Promotions tab in Gmail, and land in spam at a regional provider. Without pre-send testing, you have no visibility into that fragmentation.
Deliverability testing gives you a chance to fix issues before they affect revenue or engagement. This is especially important for companies that run multichannel or large-scale campaigns, since in such cases, even minor issues can quickly escalate.
Email providers also use sender reputation to determine whether an email is likely to be trustworthy or not. Your sender score can mean the difference between landing in the inbox, getting sent to spam, or being blocked completely.
What to fix:
- Regularly check inbox placement across different providers
- Test every new campaign before launch
- Monitor sender reputation and domain health
- Treat spam testing as a standard step in campaign preparation
GlockApps inbox placement tests show how your message performs across different mailbox environments, surfacing issues with content, sender IP, and domain reputation, blocklisting, and spam filtering.
6. Volume spikes that read as suspicious behavior.
Your sending behavior is one of the strongest indicators of your legitimacy as a sender. Mailbox providers expect consistency from your communications. If your company normally sends a few thousand emails, suddenly sending hundreds of thousands looks very different from normal behavior.
Pattern changes do not automatically cause deliverability problems, but they can increase the risk of being flagged by spam filters and lower your trust score. When it decreases, even legitimate emails are more likely to be blocked. On the other hand, consistent positive interactions from your users help build a strong domain reputation and make it easier to bypass spam filters.
What to fix:
- Increase volume gradually
- Warm up new domains or sending infrastructure
- Maintain a predictable sending schedule
- Avoid sudden campaigns to old or unengaged lists
Segment your audience into smaller groups to manage volume effectively. Instead of emailing your entire list at once, spread the emails over different times or days.
“The analysis of email deliverability reports from the past year highlights the ongoing importance of email volume, particularly during peak seasons when marketing messages surge. Our statistical findings show that larger senders achieve higher Inbox placement rates across major providers when they maintain consistent sending practices. Domain health, reputation, list quality, and recipient engagement with your email play an important role, influencing where new messages ultimately land.” — Julia Gulevich, Head of Customer Success at GlockApps and DMARKOFF, email deliverability expert with over 16 years in email marketing.
Conclusion
Email programs require continuous visibility to ensure their effectiveness. This includes monitoring authentication, actively managing list quality, checking spam filters, and maintaining stable sending behavior. When businesses cannot see what is happening across their sending ecosystem, mistakes can accumulate, leading to decreased inbox placement.
Deliverability is the baseline of email marketing. All the creativity, segmentation, personalization, and automation mean nothing if your emails don’t reach inboxes. Seeing your emails end up in the spam folder can be frustrating, especially if you follow the rules and put a lot of effort into your campaigns. The upside is that this issue isn’t permanent. While improving email deliverability is not an exact science, following best practices and avoiding common email mistakes can significantly increase your chances of success.
That’s why your email marketing workflow needs specific tools that provide structured analytics on performance, domain authentication, and sender reputation. The ultimate goal is to maintain trust with every mailbox provider each time you send an email.
