How do email experts troubleshoot customer deliverability issues?
Matthew Whittaker
Co-founder & CTO, Suped
Published 13 Aug 2025
Updated 19 Aug 2025
6 min read
Email deliverability can feel like a labyrinth, with countless variables influencing whether your messages land in the inbox or disappear into the spam folder. When a customer reports that their emails aren't being received, it often kicks off a deep dive into the technical and behavioral aspects of their sending infrastructure.
My goal is always to pinpoint the root cause efficiently, but that's not always straightforward. Sometimes, what seems like a simple delivery problem can uncover deeper issues with a sender's setup or practices. It requires a systematic approach, combining technical diagnostics with an understanding of sender reputation and recipient behavior.
We often deal with scenarios where customers believe an issue is on our end when, in reality, it resides within their own configuration or recipient-side factors. My process involves meticulously checking every potential point of failure, moving from the most common issues to the more obscure ones, to get to the bottom of why emails are not reaching their intended recipients.
Initial diagnosis and data gathering
The first step in any deliverability troubleshooting process is gathering initial data. This involves understanding the scope of the problem, whether it's affecting all recipients or just specific domains, and collecting any bounce messages or error codes available. These details provide crucial clues for where to start our investigation.
Customer perception
Customers often perceive a simple email deliverability issue as an email service provider problem, reporting that their messages simply aren't arriving. They might assume their emails are going to spam or getting lost without a clear reason, leading to frustration and requests for immediate fixes.
Expert diagnosis
From an expert's perspective, this initial report triggers a systematic investigation. We look at server logs for acceptance confirmations, analyze bounce codes, and check sender reputation metrics. Often, the issue is on the recipient's end, such as an overly aggressive spam filter or a misconfigured inbox, or with the sender's authentication. We frequently diagnose deliverability issues by collecting raw email headers and examining them for clues.
One of the first things I do is review the customer's sending history and any recent changes they've made to their email campaigns. We look for sudden spikes in volume, new content, or changes to their recipient lists. I also try to replicate the issue by sending test emails to various domains, including common providers like Yahoo and Gmail, to observe behavior directly. This initial phase helps us narrow down the potential causes significantly.
Technical deep dive: Authentication and reputation
Once initial data is collected, my next focus is on technical configurations, particularly email authentication. We meticulously check the sender's DNS records for proper SPF, DKIM, and DMARC setup. Misconfigurations here are incredibly common and can significantly impact deliverability, leading to messages being rejected or routed to spam folders.
I often find issues where the SPF record is missing necessary includes, the DKIM signature is invalid, or the DMARC policy is either too strict or not configured correctly to allow for proper reporting and enforcement. Rectifying these issues is fundamental for building and maintaining a strong sender reputation. Without proper authentication, even legitimate emails can be treated as suspicious. Our guides on fixing SPF and DMARC settings are frequently consulted at this stage.
Alongside authentication, I scrutinize the sender's IP and domain reputation. This involves checking various blocklists (or blacklists) to see if the sending IP or domain has been listed. A listing on a major blocklist can severely impact deliverability across many ISPs. I also delve into the history of their sending patterns to understand if sudden volume changes or sending to unengaged users have damaged their standing with mailbox providers.
Content and engagement analysis
While technical setups are critical, content and engagement play an equally vital role. I often review the actual email content for characteristics that might trigger spam filters. This includes examining subject lines, body text, image-to-text ratio, and the use of links. Even minor content issues can sometimes explain why emails aren't reaching the inbox.
Good practices
Bad practices
Content relevance: Delivering valuable, personalized content that recipients expect.
Spammy keywords: Using excessive promotional terms, all caps, or unusual characters.
Poor list quality: Sending to purchased lists, old lists, or those with high bounce rates.
Engagement monitoring: Tracking open and click rates, and segmenting inactive users.
Ignoring feedback loops: Not promptly processing unsubscribes or complaint reports.
User engagement metrics provide clear signals about sender reputation. Low open rates, high unsubscribe rates, and increased spam complaints tell ISPs that recipients don't value the emails. These metrics are a strong indicator of declining deliverability and are crucial to address. Improving engagement often means segmenting lists, personalizing content, and sending emails only to those who truly want them.
I often advise customers to maintain a healthy email list and use double opt-in processes. This proactive approach minimizes bounces and complaints, which are significant factors in how ISPs view a sender. Understanding the subtle nuances of recipient engagement is a core part of diagnosing and solving email delivery issues.
ISP feedback and advanced strategies
After analyzing technical configurations and content, I turn to ISP-specific feedback mechanisms. Tools like Google Postmaster Tools provide invaluable insights into sender reputation, spam rates, and authentication errors from the perspective of major mailbox providers. This data helps confirm our suspicions and guides remediation efforts.
Proactive monitoring and remediation
Monitor daily: Regularly check domain and IP reputation via Postmaster Tools and other resources.
Address blocklistings: If a domain or IP is on a blocklist, initiate delisting requests promptly. Understanding how email blacklists work is essential.
Implement feedback loops: Use FBLs to automatically remove recipients who mark emails as spam.
In some cases, especially with persistent issues or delivery to specific business domains, direct communication with the ISP's postmaster team may be necessary. This is often a last resort, as most issues can be resolved through careful diagnosis of the sender's own setup and practices. My approach is to empower customers with the knowledge and tools to maintain optimal deliverability themselves.
Views from the trenches
Best practices
Maintain meticulous records of mail logs and bounce messages to quickly identify patterns and pinpoint issues.
Consistently monitor DMARC reports to spot authentication failures and potential abuse of your domain.
Prioritize email list hygiene by regularly removing unengaged or invalid email addresses.
Segment email campaigns based on recipient engagement to send only to active and interested subscribers.
Proactively register with ISP postmaster tools to access detailed insights into your sending reputation.
Common pitfalls
Relying solely on anecdotal customer reports without delving into technical diagnostics like email headers and DNS records.
Neglecting to warm up new IPs or domains, leading to sudden drops in deliverability and poor reputation.
Sending emails to purchased lists or old, unengaged contacts, which can trigger spam traps and increase complaints.
Ignoring feedback loop reports, missing critical signals about recipient spam complaints and engagement.
Failing to implement or properly configure all three email authentication protocols: SPF, DKIM, and DMARC.
Expert tips
Sometimes, the simplest solution is for the customer to check their spam folder, even if they insist they have.
If a customer reports an issue, always confirm whether the email was accepted by your mail servers first.
Be prepared for clients who are reluctant to make necessary changes on their end, even when presented with clear evidence.
Leverage the insights from ISP postmaster tools, as their data is often more accurate than perceived deliverability.
Document all troubleshooting steps and findings, as this helps track recurring issues and demonstrates due diligence.
Expert view
Expert from Email Geeks says they had a big victory when Yahoo Postmaster confirmed that mail was accepted by their servers, validating their assessment that the customer needed to contact their own ISP.
2020-10-08 - Email Geeks
Marketer view
Marketer from Email Geeks says they also had a similar experience where the client's URL in the email content was causing deliverability issues, and the provider confirmed it after the client was reluctant to remove it.
2020-10-08 - Email Geeks
Navigating the email deliverability landscape
Troubleshooting email deliverability issues is a multifaceted challenge that demands a blend of technical expertise, data analysis, and a good understanding of ISP policies. My approach always centers on a systematic investigation, starting with the basics of data collection and moving through authentication checks, content and engagement analysis, and finally leveraging ISP feedback. By addressing these areas comprehensively, we can typically diagnose the problem and provide actionable steps to restore optimal email delivery.
