What greylisting is?
Greylisting is antispam technique, be done using the mail message agent (MTA). Using this method, e-mails with unknown IP addresses are rejected on the first attempt at delivery and the sending server must be reused sending – which happens with a standard e-mail system.
How antispam greylisting method works
When an email address sends mail to a mail server, the server may respond correctly or reply incorrectly. Two types of SMTP server errors are available: permanent, completely rejecting the message and supported by further attempts to deliver it, and temporary, which allows you to try to resend the email at a later time.
If the mail server receives an email from an IP address that is “unknown” for the server, the mail server serving greylisting temporarily rejects it intentionally, but only on the first delivery attempt. The sender’s mail server then receives a reply with information about a temporary message delivery error and a request to try sending the same message again.
If the attempt to deliver the e-mail is repeated, the address will be marked as trusted for some time and the message will no longer be rejected. In this case, the mail will be delivered correctly.
The effectiveness of the greylisting antispam method
This method works as an anti-spam system for a simple reason: spammer servers do not support this type of error – the verses saying that a given server has temporarily refused to accept the parcel are ignored, as a result of which the sending of spam is not repeated.
Servers sending spam usually try to send as many emails in the shortest possible time – the number of emails sent is important, so these servers do not pay attention to the correct handling of smtp protocols and usually ignore the mail server’s request for re-sending. Re-sending is only performed by correctly configured mail servers that send real e-mails.
Greylisting has several configuration options, e.g. ready lists of known email providers that can be excluded from a deliberate error response and re-sending request.
Within the server that supports greylisting, you can freely set the time when re-sending is expected and the time for which the e-mail server that re-sends is added to the temporary white list.
Greylisting is a relatively effective method to get rid of a large portion of spam, mainly one that is sent in bulk. A disadvantage, however, is the fact that there is a delay of several to several minutes in receiving the first email from an external mail server. Subsequent emails from the same server arrive without delay.
Greylisting has become a popular and standardized solution in the fight against spam – this is used by many companies on mail servers.
More information you can read on the Wikipedia page: https://en.wikipedia.org/wiki/Greylisting