What is Greylisting and Anti-Greylisting Technology?
Multiple filters have been created to keep the recipients safe from spam messaging. Blacklisting, whitelisting, IP blocking, domain blacklisting and other tools have been doing a reasonable job but a huge problem faced by a majority of users is that even some legitimate emails end up in spam folder as the filters are not perfect enough. In such cases, greylisting seems to be a perfect way, bringing quality emails straight to your inbox and blocking spam. So what is greylisting?
What is Greylisting? How Does It Work?
Greylisting is a new popular anti-spam technology, similar to whitelisting and blacklisting, deployed by mail servers to prevent spam.
An MTA (Mail Transfer Agent) using greylisting will block suspicious emails by giving a “temporarily reject” error. In such a situation if the email is legitimate, the originating server will try sending the email again within a time-frame specified in the RFCs. This process goes on till the email is finally accepted by the recipient server.
On the contrary, servers for mass email spam do not have such sophisticated features. Thus the email once rejected is not resent, leaving the inbox safe from the spams.
Example: [email protected] sends an email to [email protected] Kashe is using greylisting. In the first trial to deliver Sasha’s message, Kashe’s mail server denies it, shows the error message “temporarily reject” and takes a note of it. Sasha is using a normal email server which tries resending the email after a few minutes. When Kashe’s mail server sees the second attempt it accepts and delivers the message to the inbox. So Kashe receives the email with a delay but is protected from any kind of spam.
There are multiple types of greylisting amongst which time-based greylisting, as mentioned in the example, is the most common. Each mail server has a different time for which they greylist the sender, normally between 1 and 15 minutes per email.
Simple Precautions To Avoid Greylisting?
Maintain a good sender and IP reputation
Use a reliable domain
Provide the option of instant unsubscription to the users
Use a real sender’s name while signing up your email
Avoid using spammy words like - “Buy”, “Hurry”, “Free” etc
Format the heading and content of the message according to the HTML standards and RFC 5322 (internet message format)
What is Anti-Greylisting?
Some mail servers adopt the anti-greylisting technology to adjust to greylisting. In the process of validation, anti greylisting is adopted to get deliberate pauses between multiple validation trials to simulate the behaviour of a well-configured email server and to prevent the IP blocking.
When an email is rejected for the first time, the anti-greylisting tools try to resend it after a longer period of time to minimise the number of rejections.
This may delay the deliverability of the emails slightly but adds on to the efficiency by reducing the overall number of tries to get the emails delivered.
Different mail servers have a different time they greylist the sender for, which is normally between 1 and 30 minutes. Clearout deploys breakthrough anti-greylisting technology. Clearout’s anti-greylisting algorithm can determine the real status of email addresses without compromising on the turnaround time. The anti-greylisting process may take a little more time than usual but it increases the number of valid emails and reduces the ‘unknowns’ in the result, making the extra time worth it.
Blacklist vs Greylist
The difference is simple, senders on a blacklist will never be allowed to enter your inbox regardless of how many attempts they make. On the other hand email addresses that are greylisted are held for moderation and will be allowed on further attempts.
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