What causes high unknown rates when validating purchased email lists and are they effective?
Michael Ko
Co-founder & CEO, Suped
Published 30 May 2025
Updated 19 Aug 2025
8 min read
When you invest in an email list and then run it through a validation service, encountering a high unknown rate can be incredibly frustrating. It means the validation tool couldn't definitively tell you if an email address is valid or invalid. This ambiguity often signals deeper issues with the list's quality, particularly when it comes to purchased data.
Many marketers mistakenly believe that validating a purchased email list will somehow cleanse it and make it effective for their campaigns. However, this is a common misconception that can lead to significant problems. The nature of how these lists are acquired, combined with the technical challenges of email validation, often results in poor deliverability outcomes and wasted resources.
High unknown rates are a symptom, not the core problem. They highlight the fundamental flaws of relying on non-permission-based email acquisition strategies. Even after validation, these lists rarely perform as expected and can severely damage your sender reputation.
The inherent issues with purchased email lists
Purchased email lists are fundamentally flawed from the start. Unlike lists built through organic opt-ins, these addresses are gathered without the explicit consent of the recipients. This lack of permission is the root cause of many deliverability issues and is the primary reason why these lists are largely ineffective. Recipients on purchased lists have no expectation of hearing from you, leading to low engagement, high spam complaints, and significant reputational damage.
Often, these lists contain outdated, misspelled, or non-existent email addresses. More critically, they are frequently riddled with spam traps, which are email addresses used by mailbox providers and blocklist (or blacklist) operators to identify senders of unsolicited mail. Hitting a spam trap can swiftly lead to your domain or IP being added to a blacklist or blocklist, drastically impacting your ability to reach the inbox. Even pristine spam traps, which were never valid email addresses, can be present in these lists. This makes the concept of a cold email campaign with a purchased list a risky endeavor.
Furthermore, many email service providers (ESPs) have strict policies against using purchased lists. If detected, your account could be suspended or terminated, leading to severe disruptions in your email marketing efforts. The perceived short-term gain of a large list is quickly overshadowed by the long-term damage to your sender reputation and overall deliverability. This is why you often see high deliverability issues with purchased email lists.
Why email validation tools return high unknown rates
When an email validation tool returns an unknown status, it generally means it couldn't conclusively determine if the email address exists. This can happen for several technical reasons, and it's particularly prevalent with purchased lists because they often contain addresses from domains that employ stringent anti-spam measures or are simply poorly maintained. Mailbox providers such as Yahoo are known for this.
Common reasons for high unknown rates include:
Server timeouts: Some email servers, especially corporate or B2B servers, might not respond to validation queries within the set timeframe or may deliberately delay responses to deter spammers. This often leads to a timeout, resulting in an unknown status.
Greylisting: This is an anti-spam technique where a mail server temporarily rejects an email from an unknown sender, requesting that the sending server try again after a short delay. Validation tools typically won't retry, leading to an unknown result. You can learn more about email greylisting
ISP blocking: Some internet service providers (ISPs) actively block IP addresses used by email validation services, especially if those IPs are frequently probing their servers. This prevents the validator from getting a definitive answer.
Accept-all domains: These domains are configured to accept all incoming emails, regardless of whether the specific address exists. Validation tools cannot verify individual addresses on such domains, marking them as unknown or accept-all.
While validation services provide an essential layer of email list hygiene, they are not a magic bullet for poor quality, purchased lists. The inherent unreliability of these lists means that a significant portion will remain unverified or result in high bounces even after cleaning. This is why you often see high unknown rates with purchased lists, signaling their underlying poor quality rather than a flaw in the validator itself.
The false promise of effectiveness
Despite efforts to validate them, purchased email lists rarely deliver on their promise of quick returns. The primary reason is a lack of consent and relevance. Recipients haven't opted in to receive your emails, making them unlikely to engage positively. This leads to extremely low open rates, minimal click-throughs, and disproportionately high bounce rates, even after validation.
Mailbox providers actively monitor sender behavior. High bounce rates and spam complaints, both common with purchased lists, signal to providers that you are sending unsolicited mail. This negative feedback loop quickly degrades your sender reputation, making it harder to reach the inbox for even your legitimate, opted-in subscribers. In essence, a purchased list can poison your entire email program.
The risks of purchased lists
Reputation damage: High bounce rates, low engagement, and spam complaints from purchased lists will severely harm your domain and IP reputation.
ESP account termination: Most reputable ESPs have policies against purchased lists. Detection can lead to account suspension.
Legal implications: Sending unsolicited commercial email can lead to legal issues under regulations like CAN-SPAM or GDPR.
Even with verification, the underlying issue of non-consent remains. Marketing to individuals who haven't expressed interest in your content is a losing battle. It wastes your marketing budget and can create negative brand associations. The perceived benefit of a quick, large audience is often an illusion, as the real impact is a diminished ability to reach your actual, engaged audience.
Understanding validation statuses
Email service providers are increasingly vigilant about list quality. This table highlights typical verification outcomes and what they mean for your sending strategy:
Validation Status
Description
Impact on deliverability
Valid
Email address is confirmed to exist and accept mail.
Good. High deliverability.
Invalid
Email address does not exist or has a syntax error. Will result in a hard bounce.
Bad. Leads to increased bounce rates and hurts sender reputation.
High risk of spam traps and non-existent addresses, severely damaging sender reputation and deliverability.
Often violate ESP terms of service, leading to account suspension or termination.
Organically grown lists
Explicit consent: Recipients actively choose to receive your emails, resulting in higher engagement and conversions.
Lower risk of bounces and spam complaints, preserving a positive sender reputation and ensuring higher inbox placement.
Aligns with ESP best practices, fostering a strong, long-term relationship with your email platform.
Ultimately, the effectiveness of an email list hinges on consent and relevance. While email validation helps identify problematic addresses, it cannot transform a fundamentally flawed purchased list into a high-performing asset. Focus your efforts on building your list organically through valuable content and clear opt-in processes. This approach ensures you are reaching an engaged audience, protecting your sender reputation, and maximizing your email marketing return on investment.
Views from the trenches
Best practices
Always prioritize building your email list organically through opt-in forms and valuable content to ensure recipient consent.
Regularly clean your email lists to remove inactive subscribers, hard bounces, and known spam traps to maintain high quality.
Implement DMARC, SPF, and DKIM to authenticate your emails and enhance your sender reputation.
Common pitfalls
Relying solely on email validation services to make purchased lists effective, as they don't address the lack of consent.
Ignoring high unknown rates or accepting them as normal, which can mask underlying deliverability issues and reputation risks.
Failing to monitor engagement metrics (opens, clicks, complaints) when using any list, as these are true indicators of list health.
Expert tips
Monitor Google Postmaster Tools and other feedback loops for insights into your sender reputation.
Segment your list to send targeted content, improving engagement and reducing spam complaints.
A/B test your email content and subject lines to optimize engagement and inbox placement.
Expert view
Expert from Email Geeks says that just cleaning a purchased list does not make it a good list, and many validation platforms will not knowingly clean purchased lists, with most ESPs refusing them entirely.
2021-10-06 - Email Geeks
Marketer view
Marketer from Email Geeks says that if email validation tools return an unknown status, it could be due to B2B domains or the validation attempts being blocked by ISPs.
2021-10-07 - Email Geeks
Moving towards sustainable email marketing
While purchased email lists might seem like a shortcut to a larger audience, they come with significant risks, including high unknown rates during validation and minimal effectiveness in achieving marketing goals. These unknown rates are often a red flag, indicating underlying issues such as server blocking, greylisting, or the presence of unverified B2B addresses, all of which compromise deliverability.
The core problem with purchased lists is the absence of recipient consent, leading to poor engagement, high spam complaints, and a damaged sender reputation. Email service providers are stringent about list quality, and using purchased lists can lead to severe penalties, including account termination. Instead of chasing fleeting numbers, focusing on building a clean, permission-based list through organic methods is key to long-term email marketing success. This approach ensures high engagement, protects your brand, and fosters a healthy sender reputation and optimal inbox placement.
