What is Greylisting?
For email security professionals they will know about greylisting and how this is similar to a blacklist. But unlike a black or whitelist this is a temporary listing making the sender send the message again to prove they are real.
How does Greylisting work?
Greylisting is a extremely clear and basic way of stopping spam coming into your business. I say this is a simple protection method as it works on the sending address and the sending server address only.
If a sender sends you an email for the first time this message will be bounced back with a temporary email error message to the sending server only, that server will be informed to try again in X minutes. Once the same server tries to send the message again the system will see this attempt and give it the all clear and allow receipt of the message.
As mentioned above, this is not just the senders address, but also the senders server address that is looked at. So if a company has two different sending servers for email then the message has to come from the same server else it will be bounced again.
Once the Greylist has been accepted this is added to a temp database for a period meaning emails from that combination will no longer be greylisted until the period has expired.
The reason this works so well is spammers generally do not use servers to email, they use scripts and other methods. As these methods do not store information and do not have the ability to retry once greylisting happens the message never gets through.
What’s the problem?
The issue is actually down to progression of email systems and trying to make them accessible 24/7. Most companies and online services like Microsoft’s Office 365 or Google’s GSuite have multiple outgoing servers to get around blacklists, server outages etc.
Having these multiple outgoing servers resolves many issues, but with greylisting this could delay emails by days as it bounced from one sending server to the other.
Conclusion
Greylisting is a wonderful idea which stops a vast amount of spam with not much overhead to spam systems. The problem is, with new services and the need for email now; this technology is showing it’s age.
Personally I would like to see this technology rise like the phoenix and once again help fight spam, but for the moment this technology is not used by us.