Knowledge Base

When greylisting is enabled for a domain, the first time Katharion sees a message from a certain sender to a certain recipient, Katharion asks the sending mail server to try again. Legitimate mail servers handle this temporary deferral by simply retrying the message a few minutes later (the interval varies based on the sending mail server). Katharion then accepts that message on the second attempt, as well as all future messages from that sender to that recipient.  Since many spammers use software that will only make the initial attempt and will not retry, the greylisting effectively blocks a substantial quantity of junk email.

Greylisting improves the level of spam detection but it introduces a latency element the first time that a person receives a message from a particular sender. Once the sender has successfully passed the first retry request, it will no longer be tested again.