What Is Email Greylisting And How It Stops Spam Email

Ever since first spam appeared in 1978, email providers have been looking for ways to protect its clients from such messaging. Blacklisting and whitelisting have been doing a more or less reasonable job to keep the recipients safe. IP blocking, domain blacklisting, and many other tools have been created in the past decades.

One of the most successful approaches is email greylisting (or graylisting). It’s an easy way to check if the sender is a spam producer and keep your mailbox safe from the unwanted messages.

A big problem the majority of users face is that legitimate emails end up in the spam folder due to filters not being perfect enough. Greylisting seems to be a good way to fight such a problem, bringing high-quality emails straight to your mailbox and getting rid of spam. So what is greylisting?

What Is Email Greylisting?

Greylisting is a fairly simple way to figure out whether the sender is respectable or not. Recipient mail servers don’t accept any of the incoming emails, telling the sending server to try re-sending their messages again within a certain time frame.

A legitimate email server will analyze such a message and try again as requested. Once it tries again, you receive the message to your inbox. However, a server sending spammy emails won’t try again, thus leaving the email out of your inbox.

How Does Email Greylisting Work?

When an email server gets a message from an unknown sender, it’s blocked by greylisting while the server’s IP address and sender and recipient addresses are cached. The sending server gets a block, which looks like a temporary 451 error. The sending mail server views this message as a temporary delay and tries to resend the message within the time frame specified in the RFCs.

When the message is resent, its IP and address information is discovered in the greylisting cache, and the message gets the green light to appear in the mailbox. The information stays in the cache for 24 hours. So senders who don’t send you emails frequently would have to go through the greylisting process again.

Each mail server sets a different time for the duration of which it requires the message to be resent. Usually, it’s between one and thirty minutes. The default value for most servers is 15 minutes. Even though greylisting delays the delivery speed of the email, it filters out the majority of spam messages, making your email experience much better.

If you are wondering how to stop spam email, greylisting is an excellent option.

What is Anti-Greylisting?

In order to adjust to greylisting, some mail servers adopt anti-greylisting technology. After the email is rejected for the first time, they try to resend it after a long period of time (about 30 minutes) in order to minimize the number of rejections.

Anti-greylisting may delay emails slightly because the server which exercises greylisting may be ready to receive the email earlier than in 30 minutes. However, it reduces the overall number of tries you have to make to get your email delivered. Overall, this method is highly efficient. 

Email Greylist vs Blacklist

The differences between greylist and blacklist are simple. The email senders on your blacklist will never be allowed through regardless of the number of attempts. The emails that are greylisted are held for moderation. Meaning they are blocked only if the sending server doesn’t try to redeliver the message.

In case you’ve accepted emails from a certain sender address before, it won’t get greylisted and end up in your mailbox immediately.

Email Greylisting vs SPF

Greylisting and SPF are both spam-fighting techniques. There is no need to choose between both options – they can be used together.

For example, some domains use several mail servers to send mail. Each mail server can be used for each next delivery attempt. In such a case, the greylisting process can be too time-consuming because each attempt from a new server will be greylisted separately.

This problem can be solved by using the Sender Policy Framework for such domains that make their SPF data available.

Greylisting is an efficient way to fight spam even though it comes with certain nuisances. By utilizing this tool together with other approaches, it’s possible to fight the majority of today’s spam.

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