What is Greylisting And Why It Should Be Enabled?
Greylisting is a procedure for delaying emails from unknown senders. When an email arrives, Greylisting causes the server to return a message that says, busy at the moment try again in some time. Valid Mail Transfer Agents, like Exim, automatically retry many times. This retry time can be between several minutes to several days. Invalid Mail Transfer Agents usually give up and move to the next spam target. WHM uses these retry attempts as a method to isolate good emails from bad ones.
cPanel Greylisting daemon also known as cpgrey, runs at SMTP receipt time. It means everything happens before any real data is sent. The cpgrey daemon looks for three things or triplets.
- a source IP address
- a source email address
- a destination IP address
If this combination is not seen in a set time frame that is configured in WHM. cpgrey will defer all emails from this triplet for a set initial block time set in WHM. After the initial block time has expired, the system will accept emails from the triplet until the max block time has expired. So we know Greylisting, let check why it should be enabled by looking at advantages and disadvantages.
Advantages Of Greylisting:
- Effective Spam Reduction: Greylisting is highly effective in reducing spam emails, as it filters out a significant portion of spam by targeting senders who do not retry delivery.
- Resource Efficiency: By temporarily rejecting emails, greylisting helps distribute the load on email servers, improving performance and reducing resource usage.
- Enhanced Email Security: Greylisting adds an extra layer of protection by filtering out suspicious or illegitimate email sources, improving overall email security.
- Adaptive Defense: Greylisting can adapt to new spamming techniques and patterns, making it effective against evolving spamming tactics over time.
- Compatibility with Other Filters: Greylisting can complement other spam filtering techniques, such as content filtering and blacklisting, further enhancing email security.
Disadvantages Of Greylisting:
- Delayed Email Delivery: Greylisting can introduce a delay in email delivery, as legitimate senders must retry their emails after the initial rejection. This delay can sometimes be noticeable and may inconvenience time-sensitive communications.
Initial False Positives: Greylisting may initially reject legitimate emails from unknown senders, resulting in false positives. However, legitimate senders are expected to retry and deliver the email in subsequent attempts.
- Administration and Management: Configuring and managing greylisting settings may require technical knowledge or administrative effort, especially when fine-tuning parameters such as the greylisting delay period or allowlisting rules.
- Incompatible with Real-Time Communication: Greylisting may not be suitable for real-time communication scenarios where instant email delivery is critical, as it can introduce delays that impact real-time conversations or notifications.
- Greylisting has a couple of associated problems: For greylisting to work effectively, the IP address of the sending SMTP server must remain unchanged throughout the email delivery process.
When the sender’s IP address SMTP server changes, the recipient’s SMTP server will consider the incoming email as unknown, resulting in the email being added to a greylist.
In certain cases, the delivery of emails can fail due to incorrect implementation or configuration of the sending mail server. The email will not be delivered if the request to resend it is not complied with by the sender’s mail transfer agent. So we know What is Greylisting And Why It Should Be Enabled.
Check out the knowledge base article on How To Enable or Disable Greylisting In WHM.
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