How to diagnose and improve email deliverability and open rates?
Michael Ko
Co-founder & CEO, Suped
Published 11 May 2025
Updated 19 Aug 2025
8 min read
Email marketing is a powerful tool, but its effectiveness hinges on one crucial factor: deliverability. It doesn't matter how well-crafted your email is if it never reaches the recipient's inbox. Many marketers experience frustratingly low email open rates, often assuming the content or subject line is the sole culprit.
While an engaging subject line is vital for enticing opens, a significant drop in open rates often points to deeper issues. Your emails might be getting delayed, routed to the spam folder, or even blocked entirely by internet service providers (ISPs). This can happen without any explicit bounce messages, making diagnosis challenging.
Identifying the root cause of deliverability issues requires a systematic approach, combining technical checks with content and engagement best practices. This guide will walk you through how to diagnose these problems and implement strategies to consistently land your emails in the inbox, ultimately boosting your open rates.
Initial diagnosis and common pitfalls
When faced with a sudden dip in open rates, it's natural to question everything about your campaign. I once experienced a drastic drop from 26% to 7% for a major campaign, despite similar audience criteria and a 99%+ delivered rate. This immediately suggested that the emails were indeed reaching mail servers, but were likely being shunted away from the primary inbox.
My initial thoughts ranged from reporting delays to a poor subject line. However, a crucial first step is to verify whether emails are being delayed or delivered to spam folders at major ISPs. You can check individual email headers to see if a specific message experienced delays at the recipient's mail server. Tools like the Google Message Header tool can help dissect header information and reveal delivery paths.
Beyond technical delays, content choices can also trigger spam filters. In my case, an animated countdown timer from a new third-party tool and a "flash sale" subject line, despite being used in a B2B context, contributed to emails landing in spam. It's easy to overlook how seemingly innocent elements or even an ESP's rate limiting can impact inbox placement. Your ESP (like Marketo) might also be slowing down sends to avoid deferrals, especially for larger campaigns.
It's essential to look for an ISP-level breakdown in your email service provider's (ESP) reporting. Without this granular data, it is difficult to pinpoint which major mailbox providers, like Gmail, are causing the drop. If your largest recipient base, such as Gmail users, shows significantly lower engagement, that's a strong indicator of an ISP-specific deliverability problem.
Building strong technical foundations
The cornerstone of good email deliverability lies in robust email authentication. This includes setting up SPF, DKIM, and DMARC records for your sending domains. These protocols verify that your emails are legitimate and prevent spoofing, signaling to ISPs that you are a trustworthy sender. Improper configuration of these records is a common reason why emails end up in spam.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) is particularly powerful as it provides instructions to receiving mail servers on how to handle emails that fail authentication checks, and it provides valuable aggregate reports. These reports offer insights into authentication failures, helping you diagnose issues and understand why emails might be failing to reach the inbox. We have a detailed guide on understanding and troubleshooting DMARC reports.
Your IP and domain reputation are also critical factors. ISPs assign a reputation score based on your sending practices, engagement metrics, and complaint rates. A poor reputation can lead to emails being blocked or throttled. Warming up new IPs, especially when sending large volumes, is crucial to build a positive sending history. If your domain reputation is low, it can take time to recover domain reputation.
DMARC policy example
To implement DMARC, you'll need a DNS TXT record that looks similar to this. Start with a relaxed policy like p=none to monitor results before enforcing quarantine or reject policies.
This example sets the policy to "none" (monitoring only), requests forensic and aggregate reports, and specifies where to send them. This allows you to gather data and verify legitimate email flow before moving to stricter policies like p=quarantine or p=reject.
Content, list hygiene, and engagement
Maintaining a clean email list is paramount for deliverability. Sending to invalid or disengaged email addresses inflates your bounce rate, increases spam complaints, and can lead to your domain or IP being added to a blocklist (or blacklist). Regularly cleaning your list by removing inactive subscribers and invalid addresses can significantly improve your sender reputation and newsletter open rates. Spam traps, which are old, abandoned, or intentionally planted email addresses, are especially damaging.
Engagement metrics are closely monitored by ISPs. Low open rates, click-through rates, and high unsubscribe or complaint rates signal to ISPs that your emails are not valuable to recipients, leading to poorer inbox placement. Conversely, high engagement boosts your sender reputation. To improve engagement, segment your audience and personalize content. A study by the Radicati Group found that personalized emails can significantly increase open and click-through rates.
Your subject line and preview text are the first impressions of your email. They must be compelling and relevant to avoid being flagged as spam or simply ignored. Avoid spammy keywords, excessive punctuation, or misleading statements. A/B test different subject lines to see what resonates best with your audience. The preheader text (preview text) is equally important, as it complements the subject line and offers more context, influencing whether a recipient decides to open your email.
Content mistakes
Using spammy keywords in subject lines or body.
Purchasing email lists with unengaged or invalid contacts.
Irrelevant content that doesn't match subscriber expectations.
Inconsistent sending volume, leading to reputation spikes/drops.
Best practices for engagement
Personalize sender name and content.
Segment your audience for targeted messaging.
Optimize subject lines and preview text.
Maintain consistent sending frequency and volume.
Continuous monitoring and improvement
Proactive monitoring is non-negotiable for long-term email deliverability success. Tools like Google Postmaster Tools provide invaluable data on your domain's reputation, spam rate, feedback loop data, and authentication status specifically for Gmail recipients. Regularly checking these dashboards can alert you to potential issues before they escalate, helping you understand why your emails are going to spam.
Being listed on an email blocklist (or blacklist) can severely impact your deliverability. These lists compile IP addresses or domains that have been associated with spamming activities. If you find yourself on a blocklist, it is crucial to identify the cause, rectify the issue, and then request delisting. Monitoring various blacklists (or blocklists) is part of a proactive strategy.
Email deliverability and open rates are not static; they require continuous optimization. A/B testing different elements, from subject lines and sender names to email content and send times, can provide data-driven insights for improvement. Always analyze your performance metrics (opens, clicks, conversions) and adapt your strategy based on what works best for your audience. This iterative process ensures your email program remains healthy and effective.
Metric
Description
Impact on deliverability
Open rate
Percentage of recipients who open your email.
Lower rates indicate poor inbox placement.
Click-through rate
Percentage of recipients who click a link.
Higher rates signal engagement and content relevance.
Bounce rate
Percentage of emails that couldn't be delivered.
High rates hurt sender reputation and can lead to blocklisting.
Complaint rate
Percentage of recipients who mark your email as spam.
High rates severely damage reputation and lead to spam folder delivery.
Unsubscribe rate
Percentage of recipients who opt out.
High rates suggest content misalignment or frequency issues.
Views from the trenches
Best practices
Regularly clean your email lists to remove inactive or invalid addresses, improving overall deliverability.
Implement robust email authentication protocols like SPF, DKIM, and DMARC for domain credibility.
Gradually warm up new IP addresses or domains to build a positive sending reputation with ISPs.
Consistently segment your audience and personalize content to increase relevance and engagement.
Common pitfalls
Sending large, sudden campaign volumes without prior warm-up can trigger spam filters and delays.
Ignoring email header information, which can reveal crucial details about delivery delays or spam folder placement.
Using overly promotional or "flash sale" wording in subject lines, especially for B2B audiences.
Failing to monitor Google Postmaster Tools for domain reputation and spam rate insights.
Expert tips
Always test emails across various clients and devices to ensure proper rendering and avoid broken elements.
Actively solicit replies to your emails to signal positive engagement to mailbox providers.
Analyze open rates by ISP to identify specific deliverability issues with certain providers.
Use a consistent "from" name and email address to build brand recognition and trust with recipients.
Marketer view
Marketer from Email Geeks says: "I sent a big campaign and saw unusually low open rates. I was wondering if Gmail might be quarantining or delaying inbox placement."
July 25, 2019 - Email Geeks
Marketer view
Marketer from Email Geeks says: "My less likely theories were a delay in Marketo reporting or a terrible and non-compelling subject line."
July 25, 2019 - Email Geeks
Path to inbox success
Diagnosing and improving email deliverability and open rates is a multi-faceted task that goes beyond simple campaign adjustments. It requires a holistic view, encompassing technical configurations, sender reputation management, vigilant list hygiene, and highly relevant, engaging content. Each element plays a crucial role in ensuring your messages bypass spam filters and reach the intended inboxes.
By diligently monitoring your performance with tools like Google Postmaster Tools, maintaining clean lists, authenticating your emails properly, and continuously refining your content strategy, you can significantly enhance your email program's effectiveness. Consistent effort in these areas will not only improve your open rates but also strengthen your overall email marketing ROI.
Gmail, are causing the drop. If your largest recipient base, such as Gmail users, shows significantly lower engagement, that's a strong indicator of an ISP-specific deliverability problem.
