Greylisting explained

Hi all, From reading the threads, and talking to technical support, I still did not understand how greylisting works. Can someone get into the details ? "if an email from an unknown domain comes ": What does make a domain known or not ? And once a domain is known, does it stay known for ever ? I understand it is first rejected by my mail server, but when does it get delivered ? About the settings: Initial deferral period: (default 10 minutes) -> Is this how long an email will stay rejected before it gets delivered ? Resend acceptance period: The information button says "The number of minutes during which the email server accepts a resent email from an unknown triplet" (default 240 minutes) - -> What happens when the same email is sent within this period of time, but before the 10 minutes above. Does it reset a counter ? -> What happens when the same email is sent within this period of time, but after the 10 minutes above ? Record expiration time (default 4320 minutes) What does this do ? If you know your way around, can you give a typical scenario ? If I look at my greylist reports, I have email from: xxx.xxx email to: yyy.yy Deferred: 3 Accepted 5 Create time: A day ago Block expire time: A day ago Must retry time: 20 hours ago Record expire time: in 2 days Did this email get delivered or not ? If not, when will it be ? Thank you all !