Use of Anti-Greylisting Technology to Counter Greylisting


Before delving into Anti-Greylisting Technology, it becomes essential to understand the concept of greylisting, as anti-greylisting is meant to counter greylisting. Greylisting is an effective and widely used anti-spam technology. It is deployed by mail servers and protects email users against spam. An email (Electronic mail) is a message (mail) that gets transmitted and distributed electronically, from and to electronic devices, via a network. Email systems work on a store-and-forward model, in which email messages are accepted then delivered and/or forwarded as well as stored by email servers. The users’ electronic devices should be connected to a mail server or a webmail interface in order to send and receive emails.

To digress, in the context of servers, there are web servers which are provided by web hosting service providers to store the files of websites. Highly efficient web hosting service providers are renowned as the “Best Windows Hosting Company” or as the “Best Website Hosting Company” or as the “Top Cloud Hosting Company.”

With regard to greylisting and emails, a mail transfer agent (MTA) that uses greylisting will temporarily reject any email that originates from a sender that is not recognized by it. At this stage, if the email is licit, the server from where it originated will attempt to resend it within a time-frame that had been specified in the RFCs. This attempt continues till the email gets accepted by the recipient server. The delay in delivery that is caused, provides sufficient time to real-time blackhole lists and other similar lists for identifying and flagging spam sources.

For spam emails, an email that is rejected in this manner is not sent again and doesn’t get delivered at all. This happens because a sending Mail Transfer Agent that is illicit will usually not retry to send a spam email. Hence, greylisting provides an effective measure to ensure protection against mass email tools which are used by spammers. It delivers an easy solution that works effectively to get rid of spam emails.

How Greylisting Works?

Let us touch upon the email delivery process, prior to proceeding to know how and at which stage of the email processing, greylisting comes into effect.

The Simple Mail Transfer Protocol (SMTP) is used to transfer an email from a sender to a recipient. Firstly, an email is composed with an MUA (Mail User Agent), which can be an email application that is locally installed or a webmail interface. Next, the MUA establishes an SMTP connection with the sender’s Mail Transfer Agent (MTA) to send the email. An MTA is a software which uses SMTP to transmit emails from one computer to another. The email gets forwarded to the recipient’s Mail Transfer Agent by the Mail Transfer Agent of the sender. Finally, this mail is delivered to the recipient’s inbox, post being accepted by the agent. The delivered email gets displayed as a new message when the local inbox of the recipient is synchronized with IMAP or POP3 protocol.

Greylisting takes place at that stage when the recipient’s Mail Transfer Agent receives a mail. The receiving MTA requires certain data before it can accept an email. The IP address of the sending mail server, the email address of the sender and the email address of the recipient form this data. This data, which is called the “envelope data”, gets recorded by the Mail Transfer Agent in a list, for each incoming email. When a set of envelope data is encountered by the Mail Transfer Agent for the first time, the email gets rejected initially and an error code is returned. The sending MTA resends the same mail, post a specified period of waiting. At this stage, when the delivery of the email is reattempted, the envelope data already exists in the greylist and the email gets delivered. This contrasts with what happens when a sending Mail Transfer Agent is not legitimate, wherein no more attempts are made to deliver the email. This prevents the spam email from getting delivered.

What is Anti-Greylisting Technology?

Anti-Greylisting Technology provides a solution to evade greylisting. Equipped with information regarding how greylisting functions, it will become easier to grasp the way anti-greylisting technology works in the validation process. This technology makes it possible to ensure that deliberate pauses are maintained in between multiple validation trials. This is done for the purpose of simulating a well-configured email server’s behavior and for preventing IP blocking. In the event that an email is rejected in the first delivery attempt, anti-greylisting resends it after a period of time that is longer. This makes sure that the number of rejections are minimized. Anti-greylisting might cause a slight delay in the deliverability of the emails but its advantage is that it reduces the overall number of delivery attempts and increases the number of valid emails, to fulfil the aim that it is meant for.

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