Messages are greylisted for the first time of seeing a new server, then when the sender sends a second time, approximately 10 minutes later, this is lifted. We do not use the classical greylisting, we have large whitelists to only applying greylisting for suspicious or unknown servers.
For greylisting to work, the sender must send the mail again between 10 minutes and 8 hours, using the same triplet, ie: from the same sender from the same IP to the same recipient. (This is usually automatically done by the sending mail server if it has been setup correctly). The messages is queued on the sending server, and the system is waiting for them to re-send the message. then that particular sending server will be placed on our internal list, so future emails will come through straight away. If the 8 hrs depasses between retries then it will start again. Greylisting will also start again if we have had reports of spam coming from this IP.