What is Greylisting?

Greylisting is an effective email filtering technique used to combat spam and protect mail servers from unsolicited emails. This method temporarily rejects emails from senders that the receiving mail server does not recognize. The essence of greylisting lies in its simplicity and the basic principle that legitimate email servers will retry sending an email after a temporary failure, whereas most spam systems will not.

How Does Greylisting Work?

When an email is sent to a server that employs greylisting, the server checks three key pieces of information against its database:

• The IP address of the incoming mail server
• The sender’s email address
• The recipient’s email address

If this combination is unknown to the server or hasn’t been seen within a specified period, the server temporarily rejects the message with a try again later error. Legitimate servers, adhering to standard email protocols, will wait and resend the email after a delay. In contrast, spam systems, which often operate on volume over persistence, are less likely to retry.

When TrueList detects a greylisted email, it will retry validation after waiting for a specified period. If the email is still greylisted, it will be marked as unknown. TrueList does not charge a credit for validations with an unknown status.