What Is Email Greylisting? | How It Controls Email Spam

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Since the emergence of spam in 1978, email providers have been searching for effective ways to protect their users from unwanted messages. While blacklisting and whitelisting have been somewhat successful in safeguarding recipients, there is still room for improvement. This is where email greylisting, also known as graylisting, comes into play. Greylisting offers a simple yet effective method to determine the legitimacy of a sender, ensuring that your mailbox remains free from spam.

What Is Email Greylisting?

Email greylisting is a straightforward approach that helps identify whether a sender is reputable or not. When an email is received by the recipient’s mail server, it initially refuses to accept the message, instructing the sending server to retry sending the email within a specific time frame.

A legitimate email server, upon receiving this instruction, will analyze the message and attempt to resend it as requested. Once the server retries and successfully delivers the email, it will be received in the recipient’s inbox. On the other hand, a server sending spam emails is less likely to retry, resulting in the email being discarded and not reaching the recipient’s inbox.

By implementing greylisting, email providers can effectively filter out spam and ensure that legitimate emails are delivered to users’ mailboxes.

How Greylisting Works: Filtering Potential Spam Emails

Greylisting operates by filtering out potential spam emails during the delivery process. Let’s delve into the workings of greylisting and understand the process in detail.

To send an email from a sender to a recipient, the Simple Mail Transfer Protocol (SMTP) is utilized. The email follows a typical path:

1. The sender composes the email using their Mail User Agent (MUA), which can be a locally installed email application or a webmail interface.

2. The Mail User Agent establishes an SMTP connection with the sender’s Mail Transfer Agent (MTA), which is software on the SMTP server responsible for receiving and forwarding emails.

3. The sender’s Mail Transfer Agent forwards the email to the recipient’s Mail Transfer Agent. If the agent accepts the email, it is delivered to the recipient’s inbox.

4. If the recipient synchronizes their local inbox using the IMAP or POP3 protocol, the email will be displayed as a new message.

Greylisting comes into play during the third step when the recipient’s Mail Transfer Agent receives the email. The receiving MTA requires three pieces of data, known as envelope data, before accepting the full email:

  • The IP address of the sending mail server
  • The sender’s email address via the SMTP command “MAIL FROM”
  • The recipient’s email address via the SMTP command “RCPT TO”

Upon receiving this envelope data, the Mail Transfer Agent records it in a list called the greylist. Each entry in the greylist consists of the sender’s IP address, sender’s email address, and recipient’s email address.

When envelope data is encountered for the first time, the Mail Transfer Agent rejects the email and returns an error code, indicating a technical problem. The sending Mail Transfer Agent is then prompted to retry sending the email after a specific waiting period.

A legitimate Mail Transfer Agent will comply with the request and attempt to deliver the email again later. During the second delivery attempt, the envelope data is already present in the greylist, allowing the email to be successfully delivered.

In contrast, illegitimate sending Mail Transfer Agents typically do not retry delivery. This is where the spam protection function of greylisting comes into play. Since no second delivery attempt is made, spam emails are effectively blocked and never reach the recipient’s inbox. This process happens seamlessly, without the recipient even being aware of it, providing an elegant solution to combat spam.

Note that greylisting does have a significant drawback due to the waiting time required for the second delivery attempt, some legitimate emails may experience noticeable delays in reaching the recipient. This delay can sometimes extend to hours, causing inconvenience in time-sensitive situations, such as password reset emails.

For instance, when using the password reset function for an online service, you may not receive the requested email immediately. Repeated attempts to request the email may also prove unsuccessful. After a few hours, multiple password reset emails may suddenly arrive, but by then, the reset links contained within them may have already expired. This situation arises from the greylisting of your email address, which can result in such delays.

What is Anti-Greylisting?

In response to the implementation of greylisting, some mail servers have adopted anti-greylisting technology. When an email is initially rejected due to greylisting, these servers employ a strategy of resending the message after a longer period of time, usually around 30 minutes. This approach aims to minimize the number of rejections and increase the chances of successful email delivery.

While anti-greylisting may slightly delay email delivery, it reduces the need for multiple resend attempts by the sending server. By waiting for a longer period before retrying, the chances of the email being accepted during the subsequent attempt are higher. Overall, this method proves to be highly efficient in navigating the greylisting process and ensuring successful email delivery.

By using greylisting and adapting to anti-greylisting techniques, email providers can effectively combat spam and enhance the deliverability of legitimate emails while maintaining a high level of security for their users.

Email Greylist vs Blacklist

When it comes to managing unwanted emails, understanding the differences between greylist and blacklist can be helpful. Both techniques are used to block spam and unwanted messages, but they operate in slightly different ways.

A blacklist is a list of email senders that are permanently blocked from reaching your inbox. Once an email address or domain is added to your blacklist, any future emails from that sender will be automatically rejected, regardless of the number of attempts they make. This can be useful for blocking persistent spammers or known sources of unwanted emails.

On the other hand, greylisting works by temporarily holding emails for moderation. When an email is received from a sender that is not on your whitelist or blacklist, it is temporarily “greylisted” and the sending server is asked to redeliver the message. If the server attempts to redeliver the email, it is then accepted and delivered to your mailbox. Greylisting can be effective in blocking spam because many spammers use automated systems that don’t retry sending the email, while legitimate senders usually do.

It’s important to note that if you have previously accepted emails from a certain sender address, it won’t be greylisted and will be delivered to your mailbox immediately. Greylisting is more effective against new or unknown senders, as it adds an extra layer of verification before accepting the email.

Email Greylisting vs SPF

Greylisting and Sender Policy Framework (SPF) are two different techniques that can be used together to combat spam and improve email security.

Greylisting, as mentioned earlier, temporarily holds emails for moderation and only accepts them if the sending server attempts to redeliver the message. This can help filter out spam emails that are often sent by automated systems that don’t retry sending. Greylisting is effective for blocking spam, but it can also delay legitimate emails from new or unknown senders.

SPF, on the other hand, is a mechanism that allows domain owners to specify which servers are authorized to send emails on behalf of their domain. It works by publishing SPF records in the domain’s DNS settings, which indicate the IP addresses or hostnames of the authorized mail servers. When an email is received, the receiving server checks the SPF record of the sender’s domain to verify if the sending server is authorized to send emails for that domain. If the SPF check fails, the email may be marked as suspicious or rejected.

Using greylisting and SPF together can enhance email security and spam filtering. However, in some cases where a domain uses multiple mail servers for sending emails, greylisting can become time-consuming as each attempt from a new server is greylisted separately. To address this issue, implementing SPF for such domains can streamline the greylisting process by allowing authorized servers to bypass the greylisting checks.

Advantage of Greylisting: Effective Filtering and No User Configuration Required

One of the main advantages of greylisting is its effectiveness in filtering out spam emails while requiring no configuration from the users. When an email is accepted and passes the greylist filter on the second attempt, it is sent directly to the recipient. This is because the greylisting system records the data on the sender and places it on a whitelist.

Greylisting plays a important role in filtering out spam. In the case of a spam attempt, the email will usually not be returned because spammers have a large quantity of messages to send and do not waste their time waiting to make a new attempt if the first one failed. By requiring a second delivery attempt, greylisting can effectively push back many spam attempts, reducing the chances of spam emails reaching the recipient’s inbox.

Greylisting does not require any configuration from the users. Once the greylisting system is implemented, it automatically applies its filtering mechanism to incoming emails without any additional effort required from the users. This makes it a convenient and hassle-free tool for combating spam.

Major Disadvantage of Greylisting: Potential Waiting Times

While greylisting offers effective spam filtering, one major disadvantage is the potential for long waiting times, especially if the Simple Mail Transfer Protocol (SMTP) is incorrectly configured. This can be unfortunate in the case of an urgent email.

How to Avoid Greylisting: Tips for Senders

To avoid being greylisted as a sender, there are several tips to follow:

1. Check and maintain your reputation as a sender: Regularly monitor your sender reputation to ensure it remains positive. This can be done by monitoring feedback loops, spam reports, and bounce rates.

2. Use a reliable domain in your sender email address: Using a reputable and trustworthy domain in your sender email address can help establish credibility and reduce the chances of being greylisted.

3. Avoid using forbidden words in your emails: Some words or phrases may trigger spam filters and increase the likelihood of being greylisted. Avoid using commonly known spam-triggering words to improve deliverability.

4. Pay attention to the format and content of your emails: Ensure that your emails have a clear and concise subject line, well-formatted content, and relevant information. Avoid using excessive capitalization, excessive links, or excessive images, as these can trigger spam filters and increase the chances of being greylisted.

5. Include a clear and simple unsubscribe option: Providing recipients with an easy way to unsubscribe from your emails demonstrates good email practices and can help prevent your emails from being marked as spam or triggering greylisting.

6. Specify a real sender address: Instead of using a generic “No Reply” or similar sender address, use a real and recognizable sender address. This helps establish trust and credibility with the recipient and reduces the chances of being flagged as spam.


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