How does using uncontrolled domains for testing affect my sender reputation?

Using domains you don't control for email testing can lead to high bounce rates, which negatively impacts your sender reputation. ISPs may flag your sending domain as suspicious, leading to legitimate emails being sent to spam or blocked. There's also a risk of hitting spam traps, resulting in your domain or IP being placed on a blacklist or blocklist.

Are there any security or compliance issues with this practice?

Yes, it poses security and compliance risks. If test emails accidentally contain real or sensitive data, it could lead to data exposure and potential compliance breaches under regulations like GDPR or CCPA. There's also the risk of inadvertently aiding email spoofing or phishing if your sending patterns are seen as abnormal from uncontrolled environments.

What are the best alternatives for email testing?

It's best to use dedicated subdomains of your own domain, like test.yourcompany.com , or set up an internal email sinkhole using tools like MailHog. These methods allow you to test email functionality without impacting your live sender reputation or risking security issues.

Why can't I just use example.com for testing?

While example.com is reserved for examples and typically doesn't accept mail, its status could theoretically change. Relying on its current configuration is risky because you don't control it. There have been instances, like with corp.com , where private domains used internally later became public, creating significant security vulnerabilities. It's safer to use only domains you explicitly control.

Why should you avoid using domains you don't control for email testing? - Sender reputation - Email deliverability - Knowledge base

When building and testing email-dependent applications, it's common to reach for quick solutions for test email addresses. Many developers and quality assurance teams might use readily available public domains like test@gmail.com or user@example.com. This approach seems convenient on the surface, allowing for quick checks of email functionalities like sign-up confirmations, password resets, or notification delivery. However, this seemingly harmless practice carries significant risks that can undermine your email deliverability, sender reputation, and overall security posture. It's crucial to understand why this shortcut should be avoided and what safer alternatives exist.

The primary concern stems from a lack of control over these external domains. When you send email to an address on a domain you don't own, you have no insight into how that domain's mail servers are configured or how they handle incoming mail. This lack of visibility can lead to unexpected consequences, ranging from your test emails being rejected outright to, more critically, inadvertently damaging your legitimate sending infrastructure. Relying on such domains for testing means you're operating in a black box, with potential negative repercussions for your email program.

Impact on sender reputation

Using domains you don't control for email testing can severely harm your sender reputation, which is fundamental to successful email deliverability. When you send emails, Internet Service Providers (ISPs) and email providers like

Gmail and

Yahoo assess your sending behavior. Sending a large volume of emails to non-existent or inactive addresses on external domains can lead to high bounce rates. High bounce rates signal to ISPs that you might be sending to an unverified or outdated list, which is a common characteristic of spammers. This can quickly degrade your domain's reputation, making it harder for your legitimate emails to reach the inbox. We have a guide on why your emails are going to spam.

A critical risk is hitting a spam trap. Spam traps are email addresses used by ISPs and blocklist operators to identify spammers. They typically have no legitimate use and should never receive mail. Sending test emails to external domains increases the likelihood of encountering these traps, especially if you're generating large volumes of random addresses. Hitting a spam trap, even accidentally, can lead to your sending IP or domain being placed on a blacklist or blocklist, severely impacting your email deliverability. You can learn more about spam traps and how they work in our in-depth guide.

Furthermore, if you're sending from a new or unestablished domain for testing, the impact can be even more pronounced. ISPs are inherently cautious of new sending domains, as they are often used by spammers. Flooding these new domains with traffic to uncontrolled addresses can quickly lead to a poor reputation, making it difficult to warm up your domain for actual marketing or transactional emails. We have a dedicated article on why you should avoid sending from a newly registered domain.

Security and compliance concerns

Beyond deliverability, using external domains for testing introduces significant security and compliance risks. When you send an email, even a test email, the recipient's mail server processes it, and sometimes, depending on your setup and the recipient domain's configuration, information about your sending infrastructure can be exposed. If you're sending from a domain that isn't properly authenticated or controlled, you could inadvertently aid phishing or spoofing attempts. Spammers and phishers often attempt to send email from domains they do not control. You can learn more about this by reading this Twilio article.

One particularly insidious risk involves the example.com domain. While commonly used for examples in documentation and testing due to its reserved nature, it's not immune to potential misuse. Although example.com doesn't currently have an MX record, meaning mail can't be delivered to it, this status could theoretically change in the future. A real-world example of a similar risk is Microsoft's acquisition of corp.com. This domain, once privately owned, was problematic because many internal Active Directory configurations accidentally pointed to it, creating a severe security vulnerability that could have been exploited by malicious actors. While the direct risk with example.com might seem lower, the principle remains: never send to domains you don't fully control or understand, as their status or ownership could change, leading to unforeseen security exposures.

The risks of using uncontrolled domains

Spoofing risks: Your legitimate domain could be associated with sending patterns typical of email spoofing.
Data exposure: If test emails contain any real user data, it could inadvertently be exposed to third parties.
Compliance breaches: Violating data privacy regulations like GDPR or CCPA due to uncontrolled data flow.

Another often overlooked aspect is the legal and compliance ramifications. Depending on the nature of your test emails and the data they contain, sending to uncontrolled external domains could constitute a data privacy breach if real or sensitive information is accidentally included. This could lead to significant fines and reputational damage under regulations like GDPR or CCPA. It's a risk that is simply not worth taking when safer alternatives are available.

Practical issues and better testing methods

From a practical standpoint, sending test emails to uncontrolled domains provides unreliable results. You can't definitively know if an email was delivered, rejected, or sent to a spam folder, which defeats the purpose of testing. This lack of clear feedback makes it impossible to accurately assess your application's email functionality or troubleshoot any issues effectively. You need a controlled environment to ensure your tests reflect real-world outcomes without negatively impacting your live sending reputation.

The best practice is to set up a controlled testing environment using domains you own. This typically involves using dedicated subdomains for testing purposes. For example, instead of yourcompany.com for live emails, you might use test.yourcompany.com or dev.yourcompany.com. We have a detailed article on why you should use subdomains for email marketing.

For true functional testing, consider using email testing tools or setting up an internal email sinkhole. An email sinkhole is a mail server designed to accept all incoming email for a particular domain but discard it without storing or forwarding it. This allows your application to send emails as if to a live recipient, without any deliverability or security risks. Tools like MailHog provide a web interface to view these test emails, ensuring that the email content and formatting are correct without leaving your controlled environment. For a simple local setup, MailHog can be initiated with a Docker command:

Run MailHog with Dockerbash

docker run -p 8025:8025 -p 1025:1025 mailhog/mailhog

Prioritizing controlled email testing

In the realm of email testing, taking shortcuts by using domains you don't control, such as gmail.com or example.com, introduces a cascade of risks. These include serious damage to your sender reputation, the potential for your domain to be listed on a blocklist (or blacklist), and significant security and compliance vulnerabilities. My recommendation is always to prioritize the integrity and security of your email program.

By investing in controlled testing environments, such as dedicated subdomains or email sinkhole systems, you can ensure accurate test results, maintain a pristine sender reputation, and protect your domain from unforeseen risks. This proactive approach is not just about avoiding problems, but also about building a robust and trustworthy email infrastructure that supports your long-term communication goals.

Views from the trenches

Best practices

Always use dedicated subdomains or internal email testing tools for all testing.

Implement email authentication (SPF, DKIM, DMARC) on your testing domains.

Regularly monitor your domain's sending reputation using tools like Google Postmaster Tools.

Common pitfalls

Using public domains like gmail.com or example.com for any type of email testing.

Generating large volumes of random email addresses on external domains.

Ignoring bounce rates or deliverability metrics during testing phases.

Expert tips

Consider open-source tools like MailHog or dedicated testing services for robust functional testing.

Educate your development and QA teams on email deliverability best practices.

Ensure your test environments are completely isolated from your production sending infrastructure.

Expert view

Expert from Email Geeks says they have dozens of sinkhole domains and an internal disposable email system to manage test environments effectively.

2024-12-18 - Email Geeks

Marketer view

Marketer from Email Geeks says they initially believed sending to example.com was harmless because its MX record was configured to discard mail, assuming there was no deliverability risk.

2024-12-18 - Email Geeks