What is Email Greylisting?

Email greylisting is an anti-spam technique used by mail servers to filter incoming email messages based on the behavior of the sending server.


Suspicious emails are temporarily delayed by the receiving mail server and the sender server is instructed to rety. Legitimate email servers will typically retry several times. However, spam email servers will give up after the first blocked attempt.



How Does Greylisting Work?

Email greylisting uses a “wait and see” approach. Rather than blocking the email outright, it temporarily delays its arrival. Here’s how it works:


  1. The server temporarily rejects the email: When a mail transfer agent (MTA), the software responsible for email delivery, receives an email from an unknown sender, it temporarily rejects the message. This rejection is communicated with a specific error code (typically a 4xx code in the SMTP) indicating a temporary issue.

  2. The sending server tries again: Legitimate email servers are designed to handle temporary errors by automatically retrying the delivery after a predefined delay. This delay can range from a few minutes to several hours, depending on the server configuration.

  3. The email is successfully delivered upon retry: If the sending server retries the email delivery within the designated timeframe, the greylisting filter recognizes it as a legitimate attempt and delivers the message to the recipient’s inbox. The sender’s IP address and email details are often added to a whitelist for future messages to bypass the delay.

  4. Spam emails don’t get through: Conversely, spammers typically don’t configure their servers to retry failed deliveries. As a result, the email never reaches the recipient’s inbox.


Email Greylisting vs. Blacklisting

While both greylisting and blacklisting aim to combat spam, they differ in their approach:


  • Greylisting: This method creates a temporary hurdle by delaying emails. It’s a “wait and see” approach that relies on the sender’s behavior to determine legitimacy.


  • Blacklisting: This method completely blocks emails from specific senders or domains known for spamming activities. Blacklists are more aggressive but can also lead to accidentally blocking legitimate emails



Advantages of Greylisting

  • Reduced spam: By introducing a delay for unrecognized senders, greylisting discourages spammers who rely on high volumes of emails sent quickly.


  • Minimal user intervention: This method operates silently in the background, requiring no action from the email user.


  • High accuracy: Greylisting offers a good balance between filtering spam and allowing legitimate emails through.



Disadvantages of Greylisting

  • Delivery delays: Legitimate emails may experience a slight delay upon initial contact with the recipient’s server.


  • False positives: New or low-volume senders might face temporary delays until their reputation is established with the greylisting filter.


  • Compatibility issues: Some older email servers might not be configured to handle temporary errors by retrying deliveries automatically.



How to Avoid Getting Greylisted

Email marketers and subscription-based newsletters often encounter difficulties with email greylisting filters. This is more prevalent when emails are sent in bulk, and the recipient hasn’t saved the sender as a contact.


Here are a few ways email senders can use to avoid getting greylisted:


  • Maintain a positive IP reputation: Avoid practices that could lead to a negative sender reputation, such as sending spam or exceeding bounce rates.


  • Use a reliable domain: Sending emails through a reputable domain with a good sender score improves deliverability.


  • Include a real sender’s name: Avoid using generic or misleading sender names, as this can raise red flags for greylisting filters.


  • Allow instant unsubscribing: Provide a clear and functional unsubscribe option within your emails to comply with anti-spam regulations.


  • Craft clear email subjects: Avoid misleading or sensational subject lines often used by spammers. I.e. stop words like “buy” and “cheap”

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