How Anti-Greylisting Technology Impacts Email Validation

Email validators promise lower bounce rates, better conversions and better email deliverability.

You upload your list, click validate, and run into a problem - unknown results.

The culprit? Email greylisting.

Frequently asked questions

Email greylisting aims to protect users from incoming spam. Temporarily rejecting messages from unknown senders reduces the amount of spam that reaches the inbox or spam folder.

Email servers greylist emails to protect users from spam and malicious content. They temporarily reject messages from all unknown senders.

Legitimate senders typically configure their SMTP to re-attempt delivery before hard bouncing. This provides greylisting prevention for legitimate senders and filters out the spammers that fail to use anti-greylisting techniques.


The most commonly used email greylist check is time-based. Receiving servers reject mail from unknown senders for fifteen minutes. A well-configured email server should anticipate this and deploy anti-greylisting techniques to avoid hard bounces.

Spammers simply mass-target inboxes and will not wait out the rejection period. They’re more interested in discovering vulnerable domains rather than optimizing their deliverability. Thus, greylisting helps to filter out unwanted messages.

Greylisting is tremendously effective at stopping malicious senders. This protects you from phishing attempts, malware, viruses and other types of spam. Greylisting is a terrific solution that can protect your data.

The downside of greylisting is that it delays legitimate email content. If you need content to reach readers immediately, greylisting may negatively impact results. In certain circumstances, valid emails can take longer than fifteen minutes to hit the inbox or be lost entirely.

You’ll need to carefully assess your email needs and determine the value of protection versus the impact of possible messaging delays.

The only perceived downside to using this anti-greylisting solution is that it takes longer to receive your email validation results. However, the importance of obtaining accurate results is far greater than a temporary delay.

Remember - the time delay occurs due to the greylisting check present on an email server. Our technology allows you to determine if an email address is valid or harmful instead of sending back an “unknown” result. Spam prevention tactics will always be present, so you must use greylist avoidance strategies for your email marketing.

An email greylist check temporarily rejects messages from an email domain or IP. It’s an anti-spam technology that blocks would-be malicious senders while still allowing legitimate senders to connect with your inbox.

An email blacklist indefinitely blocks incoming messages from an email domain or IP. You might be on an email server’s personal blacklist or another list operated by public blacklisting services. If you are on a blacklist, your content will never reach the inbox.

You may find yourself on an email blacklist if:

  • Your sender reputation is low
  • You have a history of spam reports / spammy content
  • You emailed a spam trap