Greylisting

A server without implementation of greylisting would usually accept a new email right away, with no protection against spam. However, a server with greylisting implemented would first reject the same email, using an SMTP error code. If legit, the original server that had sent the first email would try to resend the email. Now, after receiving the same email twice, the server will accept this email. While greylisting was considered a powerful spam protection method a few years ago, its efficacy is now subject to debate. The following image is an infographic illustrating greylisting: