What are the best practices for email testing to avoid deliverability issues?
Matthew Whittaker
Co-founder & CTO, Suped
Published 6 Jul 2025
Updated 19 Aug 2025
8 min read
Email is a fundamental communication channel for businesses, but its effectiveness hinges entirely on one critical factor: deliverability. It does not matter how compelling your content is, how strong your call to action, or how well-designed your template is, if your emails do not reach the inbox, they cannot achieve their purpose. This is where robust email testing comes into play, acting as a crucial preventative measure against potential deliverability pitfalls.
Many common issues that lead to emails landing in the spam folder or being rejected entirely can be identified and rectified before a campaign is even sent. Without proper testing, you are essentially sending emails blindly, leaving your sender reputation and campaign performance vulnerable to unforeseen problems.
This guide will walk you through the essential best practices for email testing, covering everything from technical configurations to content optimization and the use of dedicated testing environments. By integrating these practices into your workflow, you can proactively identify and mitigate deliverability risks, ensuring your messages consistently land where they belong: in your recipients' inboxes.
Before you send: technical health checks
The foundation of good email deliverability lies in solid technical configurations. Before you even think about your email's content, you must ensure that your sending domain is properly authenticated. This involves setting up and verifying SPF, DKIM, and DMARC records. These protocols are crucial for proving to recipient mail servers (ISPs like Gmail and Outlook) that your emails are legitimate and have not been tampered with or spoofed. Without these, your emails are highly likely to be flagged as suspicious, leading to them being sent to spam or rejected outright.
A simple yet effective first test is to send emails to a variety of email addresses you own across different providers, such as GmailYahoo Mail, and Outlook. Once received, inspect the email headers to ensure all authentication checks pass. Look for SPF: Pass, DKIM: Pass, and DMARC: Pass. Any failures here indicate an urgent issue that needs to be addressed immediately. You can find specific guidelines from major providers, such as Google's email sender guidelines, to ensure compliance with their requirements.
Risks of unverified authentication
Spam folder delivery: Emails without proper authentication are often immediately flagged as spam by filters, preventing them from reaching the inbox.
Rejection: Many ISPs will reject unauthenticated emails entirely, leading to higher bounce rates and missed communication.
Reputation damage: Consistent sending without proper authentication negatively impacts your sender reputation, making it harder to reach the inbox in the future.
In addition to authentication, a clean email list is paramount. Regularly validate your email addresses to remove invalid or inactive contacts. Sending to old or purchased lists can lead to high bounce rates and encountering spam traps, both of which severely harm your sender reputation and email deliverability. Consider implementing double opt-in for new subscribers to ensure genuine engagement and minimize risks.
Optimizing content for inbox placement
Once your technical setup is robust, turn your attention to the content of your emails. Spam filters analyze various elements within your email, including keywords, formatting, and link density. Avoid excessive use of spam trigger words and phrases that are commonly associated with unsolicited emails. Test different subject lines and preheader texts to see which ones perform best and avoid those that might trigger filters. For example, using too many exclamation points or all caps can often be detrimental.
The design and formatting of your email also play a role. An email that is heavily image-based with little text can be seen as suspicious, as it might be an attempt to bypass text-based spam filters. Strive for a balanced text-to-image ratio and ensure your emails are responsive and render correctly across various email clients and devices. A poorly rendered email can lead to recipients deleting it or, worse, marking it as spam, negatively impacting your engagement rates and sender reputation.
Engagement is a strong signal to ISPs that your emails are valued. Personalize your content where possible to increase relevance and encourage opens and clicks. Make it easy for subscribers to manage their preferences or unsubscribe, as a high rate of direct spam complaints is far more damaging than unsubscribes. Regularly test different calls to action and content layouts to optimize for user interaction, which in turn boosts your overall email deliverability.
Safe email testing environments
One of the most critical aspects of testing, particularly for developers and QA teams, is to ensure that test environments do not inadvertently impact live deliverability. A common and severe mistake is sending test emails to generic or unowned addresses, such as test@hotmail.com or test@gmx.de. These addresses are often spam traps set up by ISPs to catch senders with poor list hygiene or malicious intent. Hitting a spam trap can lead to immediate blocklisting of your IP or domain, severely damaging your sender reputation.
Instead, establish dedicated, isolated test environments that mimic your production setup but send emails to a controlled location. This prevents accidental live sending and keeps your sender reputation safe. Utilizing local SMTP servers or email capture tools is a robust approach. These tools allow you to inspect emails as they would be sent, without ever leaving your internal network.
Live sending for testing
Risks: Accidental spamming of real users, hitting spam traps, damaging sender reputation.
Setup: Requires caution and manual verification of each test email delivery.
Visibility: Limited visibility into email headers and rendering across multiple clients easily.
Sandboxed testing environments
Benefits: Safe, no risk to live reputation, comprehensive inspection of emails.
Setup: Easy integration with development workflows, automated testing capabilities.
Visibility: Full access to raw email data, rendering previews, and deliverability insights.
Example of using .invalid domains in a test scriptBASH
For true isolation, leverage domains specifically reserved for testing purposes, such as .invalid or .test. These top-level domains (TLDs) are guaranteed never to resolve on the public internet, making them safe for any internal testing, including email sending. By using such TLDs, you completely eliminate the risk of accidentally sending test emails to real inboxes or hitting unsuspecting spam traps.
Ongoing monitoring and best practices
Email testing should not be a one-off event, but an ongoing process integrated into your email marketing and development cycles. Continuously monitor your deliverability metrics, such as open rates, click-through rates, bounce rates, and spam complaint rates. These metrics provide valuable insights into your audience's engagement and the effectiveness of your sending practices.
Maintain a consistent sending frequency and volume. Sudden spikes in email volume can trigger spam filters and lead to a negative impact on your sender reputation. If you need to increase your sending volume, do so gradually to allow ISPs to establish a positive reputation for your domain and IP addresses. For large sending volumes, consider warming up your IP to build trust.
Leverage tools like Google Postmaster Tools and Microsoft's Smart Network Data Services (SNDS) to monitor your domain and IP reputation. These tools provide invaluable insights into how major ISPs view your sending practices, allowing you to proactively address any potential issues. If you find your IPs on a blacklist (or blocklist), act quickly to remedy the situation and request de-listing.
Views from the trenches
Best practices
Only use mailboxes you own or have legitimate access to for email testing.
Utilize reserved top-level domains like `.invalid` or `.test` for email addresses in non-production environments.
Implement isolated test environments and local SMTP services to simulate email sending without impacting live deliverability.
Consult Postmaster Tools (like Microsoft's SNDS) to monitor your sending reputation and diagnose potential damage.
Common pitfalls
Sending automated test emails to generic or unknown addresses like `test@hotmail.com` or `test@gmx.de`.
Connecting quality assurance (QA) environments directly to live smart hosts, risking accidental spamming of real recipients.
Failing to educate development and QA teams on proper email testing protocols and the risks to deliverability.
Ignoring the potential for test emails to land in spam traps, which can severely damage sender reputation and lead to blocklisting.
Expert tips
When troubleshooting, use a dedicated mailbox you control to check email headers and ensure proper authentication.
Actively manage and clean your email lists to prevent sending to invalid addresses and avoid spam traps.
Always seek to understand and adhere to the latest sender guidelines from major mailbox providers.
Ensure all team members understand the critical impact of their actions on email deliverability.
Expert view
Expert from Email Geeks says: Using generic, unowned email addresses like test@hotmail.com for automated testing significantly risks damaging your sending reputation, as these are often spam traps.
Sep 10, 2019 - Email Geeks
Expert view
Expert from Email Geeks says: Developers should exclusively use mailboxes they own or create dummy accounts for testing to avoid deliverability issues.
Sep 10, 2019 - Email Geeks
The path to inbox success
Proactive and thorough email testing is not merely a best practice; it is a fundamental requirement for maintaining strong email deliverability. By prioritizing technical authentication, optimizing content, utilizing safe testing environments, and continuously monitoring your sending performance, you can significantly reduce the risk of emails going to spam or being rejected. Embracing these practices ensures your messages consistently reach their intended audience, fostering better engagement and achieving your communication goals.
Gmail and 
Outlook) that your emails are legitimate and have not been tampered with or spoofed. Without these, your emails are highly likely to be flagged as suspicious, leading to them being sent to spam or rejected outright.
Yahoo Mail, and Outlook. Once received, inspect the email headers to ensure all authentication checks pass. Look for SPF: Pass, DKIM: Pass, and DMARC: Pass. Any failures here indicate an urgent issue that needs to be addressed immediately. You can find specific guidelines from major providers, such as Google's email sender guidelines, to ensure compliance with their requirements.
