How can I monitor email deliverability to ensure emails hit the inbox instead of spam?
Matthew Whittaker
Co-founder & CTO, Suped
Published 29 Jun 2025
Updated 16 Aug 2025
10 min read
Ensuring your emails reach the inbox instead of spam or promotions folders is a constant challenge for anyone involved in email marketing or communication. It's not enough to simply hit send, you need a robust strategy to monitor how your emails are performing and landing. Without diligent monitoring, you could be losing valuable engagement and revenue without even realizing it.
Email deliverability is a complex ecosystem, influenced by various factors including your sender reputation, email authentication, content, and recipient engagement. Understanding where your emails are going and why is the first step toward improving your overall performance. This involves looking beyond basic metrics and diving into the technical aspects that impact inbox placement.
My goal is to help you set up a comprehensive monitoring strategy that provides clear insights into your email performance. We'll cover the essential metrics, the power of postmaster tools, the critical role of email authentication, and the importance of proactive testing and list hygiene. Let's make sure your messages consistently land where they belong: in the primary inbox.
Essential metrics for email deliverability
To effectively monitor your email deliverability, you need to track specific metrics that provide a clear picture of your sending health. While an obvious drop in open rates can signal a problem, it's just one piece of the puzzle. Focusing solely on open rates can be misleading, especially with changes like Apple Mail Privacy Protection impacting their accuracy. Instead, a holistic view of your key performance indicators (KPIs) is essential.
Beyond open rates, I always advise looking at your click-through rates. Clicks are a strong indicator of engagement, which email service providers (ISPs) value highly. If recipients are clicking on your content, it signals that your emails are relevant and desired, positively impacting your sender reputation. A decline in clicks, even if opens appear stable, can be a red flag for inbox placement issues.
Bounce rates, both hard and soft, are another critical metric. High bounce rates can quickly damage your sender reputation and lead to blocks. Monitoring these helps you identify invalid or problematic email addresses on your list, which you should promptly remove. Similarly, keeping an eye on your complaint rates is vital. Even a small percentage of spam complaints can significantly harm your standing with ISPs, prompting them to send your future emails directly to the junk folder. You can learn more about email deliverability KPIs to ensure you're tracking the right data points.
Here's a breakdown of common metrics and what they tell you:
Metric
What it indicates
Open rate
Initial engagement and potential inbox placement, though less reliable for Apple Mail users.
Click-through rate
Strong indicator of recipient interest and positive engagement signals to ISPs.
Bounce rate
High rates suggest a stale or invalid list, damaging sender reputation.
Complaint rate
Direct feedback from recipients marking your email as spam, severely impacting deliverability.
Utilizing postmaster tools and blocklists
Beyond your internal metrics, external tools offered by major email providers are indispensable for monitoring. Google Postmaster Tools (GPT) is a must-have for anyone sending to Gmail users. It provides insights into your sending reputation, spam rates, delivery errors, and DMARC authentication. Regularly checking GPT can help you identify trends and issues specific to Gmail's filtering before they escalate. It's a free service that offers invaluable data directly from the source. You can dive deeper into this with our ultimate guide to Google Postmaster Tools.
For Microsoft (Outlook, Hotmail, etc.) deliverability, their free programs, Smart Network Data Services (SNDS) and Junk Mail Reporting Program (JMRP), are equally important. SNDS provides data on your IP reputation, spam trap hits, and complaint rates for Microsoft email properties. JMRP allows you to receive copies of emails that recipients mark as junk, giving you direct insight into what content might be triggering complaints. These tools are crucial for maintaining a good sending relationship with Microsoft and understanding their filtering mechanisms.
Monitoring blocklists (or blacklists) is another non-negotiable aspect of deliverability. Getting listed on a major blocklist can severely impede your ability to reach the inbox, as many ISPs use these lists to filter out suspected spam. Regular blocklist monitoring allows you to quickly detect if your sending IP or domain has been listed and take immediate action for delisting. Understanding how email blacklists work is key to proactive management.
The impact of blocklists
Being on an email blocklist can dramatically reduce your email deliverability. It's a clear signal to ISPs that your sending practices might be problematic, leading to your emails being rejected or sent straight to spam. Prompt detection and resolution are crucial for maintaining your sending reputation.
I recommend checking major public blocklists regularly. Many services offer automated alerts that notify you if your domain or IP address appears on one of these lists, allowing for swift action. Early intervention can prevent significant damage to your email program.
The foundation of deliverability: email authentication
Email authentication protocols form the bedrock of good deliverability. SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance) work together to verify that your emails are legitimate and haven't been tampered with. Without proper implementation, even well-intentioned emails can fail to reach the inbox. I always advise my clients to prioritize getting these set up correctly.
SPF specifies which mail servers are authorized to send email on behalf of your domain. DKIM adds a digital signature to your outgoing emails, allowing the receiving server to verify that the email content hasn't been altered in transit. DMARC builds on top of SPF and DKIM, giving you a way to tell receiving mail servers how to handle emails that fail authentication (e.g., quarantine, reject, or none). Implementing a robust email authentication setup is critical for establishing trust with ISPs.
DMARC reporting is particularly powerful for monitoring. DMARC reports provide aggregate data on your email authentication results, showing you which emails are passing or failing SPF and DKIM checks, and from what sources. This gives you visibility into legitimate sending issues and potential unauthorized use of your domain (spoofing). Regularly reviewing these reports is key to refining your authentication settings and ensuring compliance. Our DMARC monitoring service can help simplify this process.
Beyond technical configurations and metrics, proactive testing and rigorous list hygiene are vital for consistent inbox placement. You can't just set up your records and forget about it. Continuous monitoring also involves actively testing how your emails perform across different mailbox providers.
Inbox placement testing tools send your email campaigns to a wide array of seed accounts across various ISPs. This helps you see exactly where your emails land: primary inbox, promotions, spam, or if they're blocked entirely. Regularly running these tests, especially before a major campaign, provides real-time insights into your deliverability effectiveness and helps you tweak content or sending practices as needed. We offer a free email testing tool to assist with this. This continuous testing helps ensure that your emails maintain a good reputation with receiving servers, proving you're a trustworthy sender.
Maintaining a clean and engaged email list is perhaps the most fundamental aspect of deliverability. Sending to old, inactive, or purchased lists introduces spam traps, increases bounce rates, and lowers engagement, all of which severely damage your sender reputation. Regularly removing unengaged subscribers and implementing double opt-in processes are critical. This proactive approach ensures you're only sending to recipients who genuinely want your emails, leading to higher engagement and better inbox placement. You can find more tips on improving your email deliverability in our guides.
Bad list hygiene
High bounce rates: Sending to invalid addresses leads to hard bounces and hurts your reputation.
Spam trap hits: Getting caught by spam traps (e.g., from old, purchased lists) triggers blocklistings.
Low engagement: Sending to unengaged subscribers signals poor content or relevance to ISPs.
Increased complaints: Recipients who don't recognize you may mark emails as spam.
Good list hygiene
Lower bounce rates: Regularly removing invalid addresses keeps your list clean.
Reduced spam trap risk: Focus on organic list growth and removal of old contacts.
Higher engagement: Engaged subscribers signal positive sending behavior to ISPs.
Improved sender reputation: Consistent good practices lead to better inbox placement.
Views from the trenches
Marketer from Email Geeks says: Grouping open and click rates by domain can be quite effective. If you see a significant decrease for one specific domain, it usually indicates an issue with that domain's filtering or reputation. This approach helps pinpoint where deliverability problems are first manifesting.
Marketer from Email Geeks says: We've noticed our deliverability tanking on Outlook and this week Gmail's sending us to spam more than promotions. We are actively discussing solutions with our team.
Marketer from Email Geeks says: Both the Gmail Primary Tab and the Promotions Tab are considered the inbox, so there's no need to try and game the algorithm to get into the Primary Tab. The Gmail Spam folder is the actual spam destination. While third-party tools offer insights into email delivery based on their probe accounts, remember these are not your actual customers' inboxes. However, they do provide a valuable indication of likely deliverability. It's also important to pay close attention to clicks, as this is a strong engagement signal. Ultimately, success depends on sending the right message to the right person at the right time, which encourages engagement and keeps your emails in the inbox.
Marketer from Email Geeks says: Deliverability service companies that use probe accounts can provide a good indication of where your mail to their generic addresses is delivered. This gives you a general sense of deliverability quality and performance.
Maintaining healthy email deliverability
Ensuring your emails consistently reach the inbox is an ongoing commitment, not a one-time setup. It requires a blend of technical diligence and strategic content planning. By regularly monitoring key metrics, leveraging postmaster tools, ensuring robust email authentication, and practicing stringent list hygiene, you create a foundation for successful email campaigns. These steps help you detect issues early and maintain a strong sender reputation.
Remember, email deliverability is about building trust with mailbox providers and your recipients. Every positive interaction, every open, and every click reinforces your sender reputation. Conversely, every bounce, complaint, or spam trap hit chips away at that trust. A proactive and comprehensive monitoring strategy is your best defense against having your emails caught in spam filters or, worse, blocked entirely.
By integrating these monitoring practices into your routine, you'll gain the visibility needed to optimize your email program continually. This ensures your valuable messages reach their intended audience, fostering better engagement and achieving your communication goals.
Apple Mail users.
Microsoft and understanding their filtering mechanisms.
Outlook and this week 
Gmail's sending us to spam more than promotions. We are actively discussing solutions with our team.
