(a) The Mail User Agent (MUA) transmits an email to the sender's mail server (P).
(b) Said server (P) relays the mail to the receiver's server (Q), which is in charge of checking the envelope data of the message: the sender's IP address and the e-mail addresses of both the sender and the receiver. If the combination of these three data has not yet been registered in the list of the receiving server (Q), the mail delivery attempt will be rejected, indicating a technical failure as the reason. The receiver's server (Q), however, will record the envelope data , that is, the mail will be included in the gray list.
(c) If it is a legitimate email, the sender's server (P) will try to send it again after a certain waiting time. Since, in this second attempt, the envelope data will already be recognized by the receiver's server (Q), the mail will be delivered. Optionally, the envelope data can then be incorporated into the whitelist of the recipient's mail server. If this happens, future emails with the same envelope data will be delivered without delay.
(d) On the contrary, if it is an illegitimately sent mail, a second delivery attempt will generally not occur. In this case, therefore, the gray list will have fulfilled its spam protection function and the mail will not reach the inbox.
The greylisting is often used in combination with other technologies of protection against spam . The Sender Agreement or, in English, Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM) as well as the DMARC (Domain-based Message Authentication, Reporting and Conformance) protocol are other effective systems to protect email traffic against to the most common forms of abuse.
In particular, gray lists work especially well when combined with other related techniques: black and white lists , that is, with the whitelisting and blacklisting methods . Next, we also present an example of the path of sending attempts to receiving servers when these mechanisms are present.