How can I diagnose and resolve email deliverability issues when emails aren't reaching intended recipients?
Michael Ko
Co-founder & CEO, Suped
Published 6 Jun 2025
Updated 16 Aug 2025
8 min read
Emails not reaching intended recipients is a critical issue that can severely impact communication and business operations. It’s a common challenge, whether you're sending transactional emails, marketing campaigns, or personal correspondence.
Understanding why your emails aren't landing in the inbox requires a systematic approach, diving beyond surface-level symptoms to uncover the root causes. Often, the issue isn't a simple server error but a combination of factors related to sender reputation, email authentication, and content.
I'll guide you through effective strategies to diagnose these frustrating email deliverability issues, offering practical steps to resolve them and ensure your messages consistently reach their destination.
Initial investigations: where to look first
The first step in troubleshooting email deliverability problems is to gather as much information as possible. Start by checking if you or your recipients are receiving any bounce messages or non-delivery reports (NDRs). These messages often contain error codes or descriptions that can point directly to the problem, such as an invalid recipient address or a blocked sender.
Next, it's crucial to confirm whether the emails are being blocked outright or simply landing in the recipient's spam or junk folder. This distinction is vital for diagnosis. If emails are going to spam, it suggests a filtering issue related to content or sender reputation, whereas outright blocking indicates a more severe problem like a blocklist listing or hard bounce. You can explore this further in our guide on how to diagnose email deliverability issues.
Basic configuration checks should also be a priority. Verify the recipient's email address for typos and confirm your SMTP server settings are correctly configured. Misconfigurations here can lead to emails not sending at all or failing at the very first hurdle.
Examining your email logs provides a detailed history of delivery attempts and outcomes, offering invaluable clues. These logs record SMTP responses, allowing you to see if the email server accepted the message, if there were any deferrals, or if a hard bounce occurred. This can help you troubleshoot message delivery issues more effectively.
Authentication and sender reputation
A cornerstone of modern email deliverability is strong email authentication. Protocols like SPF, DKIM, and DMARC verify that your emails are legitimate and prevent spoofing. If these are incorrectly set up or misaligned, recipient mail servers are much more likely to flag your emails as suspicious, even if your content is perfect. I delve into this more deeply in our article on how to troubleshoot and fix SPF and DMARC settings.
Your sender reputation, encompassing both your IP and domain reputation, is a critical factor. Internet Service Providers (ISPs) and mailbox providers use it to decide whether to accept your emails and where to place them (inbox, spam, or blocked). Factors like spam complaints, bounce rates, and sending volume all contribute to this score. Even with high IP and domain reputation reported by tools like Google Postmaster Tools, a consistently high spam complaint rate can indicate underlying content or audience engagement problems.
A high spam complaint rate, often exceeding the generally accepted risk threshold of 0.1%, is a red flag. When recipients mark your emails as spam, it signals to mailbox providers that your emails are unwelcome, negatively impacting your sender score. This can lead to future emails being filtered to the spam folder or rejected entirely, even for legitimate communications. You can find more about this in articles like how to reduce a high spam complaint rate.
Beyond internal metrics, check if your sending IP or domain is listed on any email blacklists (or blocklists). These lists, maintained by various organizations, are used by mailbox providers to block mail from known spammers. Being on a blacklist can severely impede your ability to reach recipients, resulting in emails being rejected without a bounce message. You can learn more with our in-depth guide to email blacklists.
Protocol
Purpose
Impact on deliverability
SPF
Authorizes sending IP addresses.
Helps prevent spoofing and ensures sender legitimacy.
DKIM
Digitally signs emails to verify content integrity.
Detects tampering, crucial for trust.
DMARC
Authenticates sender domain and provides reporting.
Enforces policy on failed authentication, prevents abuse.
Content, engagement, and list hygiene
The content of your emails plays a significant role in deliverability. Certain keywords, excessive links, or poorly formatted HTML can trigger spam filters. It's essential to craft clear, relevant, and engaging messages that resonate with your audience and avoid characteristics commonly associated with spam. Our article on why your emails are going to spam provides more detail.
Audience engagement and list hygiene are paramount. Sending emails to disengaged or invalid addresses increases your bounce rate and spam complaint rate, both of which harm your sender reputation. Regularly cleaning your email list by removing inactive subscribers and managing bounces is a crucial best practice. Spam traps, in particular, are hidden email addresses used by ISPs to identify senders who are not maintaining clean lists. Find out what steps you can take to resolve deliverability issues and improve sender reputation.
The type of emails you send also affects deliverability. Transactional emails, like order confirmations, typically have higher deliverability due to expected engagement. Marketing emails, especially cold outreach, carry higher risks. Mismanaging these different types can lead to deliverability issues across your entire email program.
Sending cold emails on the same domain as your corporate or transactional emails can be a major pitfall. Mailbox providers differentiate between expected, opted-in communication and unsolicited outreach. If your cold email practices lead to high spam complaints, it can inadvertently affect the deliverability of your critical corporate emails, preventing them from reaching intended recipients.
Combining email types
Sending cold outreach and corporate emails from the same domain.
High spam complaints and low engagement from cold emails affecting overall sender reputation.
Separate infrastructure
Using a dedicated domain and IP infrastructure for cold email campaigns.
Protecting primary domain reputation for transactional and corporate communications.
Advanced diagnostics and ongoing monitoring
For a deeper understanding of your email performance, DMARC reports are indispensable. These XML reports provide aggregate data on email authentication, helping you identify legitimate emails that are failing authentication, or unauthorized senders trying to spoof your domain. Analyzing these reports gives you visibility into your email ecosystem that goes beyond simple delivery confirmations.
Different mailbox providers have unique filtering algorithms and policies. What works for Outlook might not work as effectively for Gmail. It is important to consult specific sender guidelines provided by major providers like Google's sender guidelines. Monitoring your domain's reputation with Google Postmaster Tools provides insights specifically for traffic sent to @gmail.com addresses. However, it's worth noting that this data does not include information for Google Workspace accounts, as explained in the Postmaster Tools FAQs.
Continuous monitoring is key to proactive deliverability management. Relying on sporadic checks can leave you vulnerable to sudden drops in inbox placement. Implementing consistent monitoring of your authentication records, sender reputation, and blacklist (or blocklist) status allows for early detection and swift resolution of issues. This includes checking your domain reputation periodically.
When all else fails, consider using an email deliverability test. These tools can simulate sending an email to various providers and report back on its inbox placement, helping you identify if your emails are hitting spam folders or getting blocked before you even send them to your actual recipients. This is a great way to fine-tune your content and sending practices. You can learn how to run an email deliverability test with our proven checklist.
Metrics to monitor regularly
Spam complaint rate: Aim for below 0.1%.
Bounce rate: Keep hard bounces at zero, soft bounces minimal.
Open rates and click-through rates: Indicate audience engagement.
Blacklist status: Regularly check major blacklists and blocklists.
Always secure explicit consent from recipients before adding them to your mailing list.
Implement double opt-in for all new subscribers to verify email addresses.
Regularly clean your email list by removing inactive subscribers and invalid addresses.
Segment your email sending based on recipient engagement and email type.
Use a consistent 'from' name and email address that clearly identifies your brand.
Common pitfalls
Sending cold outreach from your primary corporate domain can severely damage its reputation.
Ignoring high spam complaint rates can lead to widespread inboxing issues.
Failing to configure or monitor SPF, DKIM, and DMARC records properly.
Not monitoring blacklists (or blocklists) means missing critical blockages.
Sending emails with overly promotional language or too many images.
Expert tips
Utilize Google Postmaster Tools for invaluable insights into Gmail delivery.
Set up feedback loops (FBLs) with major ISPs to receive direct spam complaints.
Warm up new sending IPs and domains gradually before sending high volumes.
Consider dedicated sending infrastructure for different email streams.
Engage with email deliverability communities to stay updated on best practices.
Marketer view
Marketer from Email Geeks says: Sending cold emails from your primary domain can cause issues, and it's best to use a separate infrastructure for cold outreach, including a different domain, to improve corporate email delivery.
November 17, 2022 - Email Geeks
Marketer view
Marketer from Email Geeks says: If you are seeing deliverability issues, ensure DKIM is enabled, as Google defaults to signing with their keys, and consider how cold emails might be affecting your sender reputation.
November 16, 2022 - Email Geeks
Proactive steps for reliable email delivery
Diagnosing and resolving email deliverability issues is a continuous process that involves technical configuration, content optimization, and diligent audience management. It requires understanding the intricate interplay of authentication, sender reputation, and recipient engagement.
By implementing robust authentication, maintaining a healthy sender reputation, and focusing on sending valuable content to engaged recipients, you can significantly improve your inbox placement and ensure your emails consistently reach their intended destinations.
Google Postmaster Tools, a consistently high spam complaint rate can indicate underlying content or audience engagement problems.
Outlook might not work as effectively for 
