How to identify the cause of declining email deliverability and improve it?
Matthew Whittaker
Co-founder & CTO, Suped
Published 5 Jun 2025
Updated 19 Aug 2025
8 min read
Experiencing a drop in email deliverability can be frustrating, leading to missed opportunities and reduced campaign effectiveness. It's a common challenge, but identifying the precise cause and implementing effective solutions requires a systematic approach. Many factors can influence whether your emails land in the inbox, the spam folder, or are blocked entirely.
When deliverability declines, it often points to issues with your sender reputation, email authentication, list quality, or even the content of your messages. Understanding where to look and what metrics to monitor is the first crucial step in getting your emails back on track and ensuring they reach your intended audience.
Initial diagnosis: assessing your metrics
The first step in diagnosing declining email deliverability is to review your key performance indicators (KPIs). You might notice a sudden drop in open rates, click-through rates, or an unexpected increase in bounce rates or spam complaints. These metrics provide immediate clues about what might be going wrong.
A rise in spam complaint rates, even if minor, is a significant indicator. While it means your emails are still hitting the inbox, recipients are actively marking them as spam, which severely damages your sender reputation. A decrease in click rates on unchanged email content, coupled with higher spam rates, suggests your emails are being filtered more aggressively, leading to poorer inbox placement. You can gain more insight into this by understanding how to diagnose and improve email deliverability and open rates.
Monitoring domain and IP reputation through postmaster tools is also essential. Tools provided by major mailbox providers, like Google Postmaster Tools or Yahoo Mail Postmaster, offer valuable insights into your sending reputation, spam rate, and authentication errors. A low IP or domain reputation is a common reason emails end up in spam.
You can also use an email deliverability tester to perform spot checks on inbox placement across various providers. This helps identify if the issue is widespread or specific to certain mailbox providers.
Technical foundations: authentication and reputation
Sender authentication is foundational to email deliverability. Incorrectly configured or missing SPF, DKIM, and DMARC records can severely impact your ability to reach the inbox. Mailbox providers rely on these protocols to verify that your emails are legitimate and not spoofed, which is a key factor affecting email deliverability.
If you're unsure about your DMARC setup, using a free DMARC record generator tool can help you create a proper record. For example, a basic DMARC record might look like this:
Beyond initial setup, consistent DMARC monitoring is crucial. It provides invaluable feedback on your authentication status, helping you to troubleshoot DMARC issues and prevent emails from going to spam due to authentication failures. This allows you to safely transition your DMARC policy to quarantine or reject as your confidence grows.
One of the most significant factors affecting deliverability is your email list quality. Sending to an outdated or poorly managed email list can lead to high bounce rates and increased spam complaints, both of which negatively impact your sender reputation. Poor list acquisition practices, such as buying email lists, are particularly detrimental.
High hard bounce rates signal invalid email addresses, which can trigger spam filters and damage your reputation. Spam traps, disguised as valid email addresses, are another major concern. Even a single spam trap hit can severely harm your sender reputation, as these are used by mailbox providers to identify senders of unsolicited mail. Regularly cleaning your email list is paramount, removing undeliverable addresses and inactive subscribers. You can learn more about how to avoid spam traps from Mailchimp.
Engagement signals are another critical component. Low open and click rates, combined with high unsubscribe rates, tell mailbox providers that your subscribers aren't interested in your content. This low engagement directly contributes to a declining sender reputation, leading to more emails being routed to spam folders. Regularly testing your deliverability can help identify issues.
Implementing a sunsetting policy for unengaged subscribers is a best practice. Instead of continuing to send to inactive recipients, gradually reduce sending frequency and eventually remove them from your active lists. This keeps your list healthy and improves your overall engagement metrics. You can also look into how to improve email deliverability when emails go to spam.
Content quality and sending practices
Beyond technical configurations and list quality, the content and sending practices of your emails play a crucial role. Email content that is overly promotional, contains too many links, or uses common spam trigger words can land your messages in the spam folder, regardless of your sender reputation. It's about balancing your message with what mailbox providers deem acceptable.
Your sending volume and frequency also matter. Sudden spikes in email volume from a previously low-volume sender can trigger red flags. New domains and IP addresses require a warm-up period where you gradually increase your sending volume to build trust with ISPs. Over-sending, even to an engaged list, can lead to increased complaints and unsubscribes.
Focus on delivering value. If subscribers consistently find your emails irrelevant or unhelpful, they are more likely to unsubscribe or, worse, mark them as spam. Personalization, segmentation, and relevant content can significantly boost engagement and positively influence your sender reputation. Consider the journey your subscribers take from opt-in to receiving your emails. Ensure the acquisition message matches the content and frequency expectations.
Finally, ensure that unsubscribe options are clear, prominent, and easy to use. Mailbox providers like Gmail are increasingly making unsubscribe buttons more accessible directly in the email client interface. If it's easier for a recipient to mark your email as spam than to find the unsubscribe link, they will choose the former, harming your sender score.
Timeline for improvement and continuous efforts
Recovering from deliverability issues can take time, ranging from a few days to several months, depending on the severity and the root cause. A minor technical fix, like an authentication error, might see improvements in weeks. However, recovering from a severely damaged sender reputation due to poor list quality or content issues will require consistent effort over a longer period. ISPs observe patterns of behavior, and it takes time to demonstrate positive changes.
It's important to approach deliverability improvement not as a one-time fix but as an ongoing process of monitoring, adapting, and optimizing. Regular audits of your sending infrastructure, list hygiene practices, and content strategy are crucial for maintaining healthy deliverability. For a deeper dive, consider reviewing this resource on technical solutions from top performing senders.
Remember, email deliverability is a dynamic field influenced by constant changes in mailbox provider algorithms and user behavior. Staying informed and proactive is the key to ensuring your messages consistently reach the inbox.
Views from the trenches
Best practices
Actively monitor all relevant metrics, including open rates, click-through rates, bounce rates, and especially spam complaint rates. These are vital for immediate insights into deliverability performance.
Implement stringent list hygiene practices. Regularly remove invalid addresses, inactive subscribers, and hard bounces to maintain a clean and engaged audience.
Prioritize email authentication protocols like SPF, DKIM, and DMARC. Proper configuration of these records is fundamental to building trust with mailbox providers.
Warm up new IPs and domains gradually. Begin with low sending volumes and slowly increase them over several weeks to establish a positive sending reputation.
Ensure clear and easily accessible unsubscribe options in all emails. Making unsubscribing simple reduces spam complaints and fosters a healthier sender reputation.
Common pitfalls
Purchasing or using old, unverified email lists. This often results in high bounce rates and spam trap hits, severely damaging sender reputation.
Ignoring authentication errors. Misconfigured SPF, DKIM, or DMARC records can cause emails to fail authentication checks and be filtered or rejected.
Failing to monitor mailbox provider feedback loops. Not addressing spam complaints or other negative feedback can lead to blocklisting and poor deliverability.
Sending inconsistent email volumes or sudden spikes. Erratic sending patterns can trigger spam filters and cause a decline in sender reputation.
Using overly promotional language or too many links. Content that appears spammy can lead to emails being filtered, even with good technical setup.
Expert tips
Regularly audit your acquisition channels. A shift in strategy can inadvertently increase spam complaints, highlighting a mismatch between expectation and content.
Understand that higher spam complaint rates often mean your emails are still reaching the inbox, but the content or frequency is causing a negative reaction, which needs addressing immediately.
Focus on the root cause rather than temporary fixes. Systemic issues in your email program may require reevaluating your business model or how you engage with subscribers.
Be patient; recovery timelines vary based on the issue. While technical fixes might be quick, rebuilding a damaged sender reputation can take weeks or even months of consistent positive sending behavior.
Analyze engagement metrics for specific mailbox providers. A significant drop in open or click rates for one provider could indicate a targeted deliverability issue that needs investigation.
Marketer view
Marketer from Email Geeks says the timeline for seeing meaningful progress on deliverability depends entirely on the issue and how quickly changes can be implemented. Simple authentication fixes are faster than issues stemming from bought lists.
2022-10-27 - Email Geeks
Expert view
Expert from Email Geeks says that determining if deliverability is declining starts with how it is being measured and what metrics are used for this conclusion. If email lists are being purchased, then recovery will be a very difficult, if not impossible, process.
2022-10-27 - Email Geeks
The path to better inbox placement
Identifying and resolving declining email deliverability requires a multi-faceted approach, combining careful metric analysis, technical configuration reviews, robust list management, and thoughtful content strategy. It's a journey that builds trust with mailbox providers and, ultimately, your subscribers.
By consistently monitoring your performance, ensuring proper authentication, maintaining a clean and engaged list, and sending valuable content, you can not only diagnose but also significantly improve your email deliverability, ensuring your messages reach their intended destination.
Gmail are increasingly making unsubscribe buttons more accessible directly in the email client interface. If it's easier for a recipient to mark your email as spam than to find the unsubscribe link, they will choose the former, harming your sender score.
