GreyListing is an anti-spam technique that makes use of a "temporary reject" status to try to identify legitimate mail servers. If the mail is legitimate the originating server will try again after a delay, and if sufficient time has elapsed the email will be accepted.

The anti-spam service analyzes the content of each message. If the anti-spam server sees email coming from a source that doesn't appear to be a regular email peer of a recipient (based on bi-directional traffic) and has certain spammy constructs or content, the message may be delayed (greylisted). The server then waits for a few attempts to send the message or a few minutes.

A normal mail server will queue a message that has been "temporarily rejected" and retry transmission in 5, 10, 15 or 30 minutes later, depending on the sending server's settings. Spam engines and bulk marketing services will not queue these messages. Often they try to force their way past GreyListing mechanisms by quickly re-sending the message a few times, then moving on to the next recipient. GreyListing is a good technique to help reduce spam originating from industrial spamvertizing sites.

In your logs you see the same message logged multiple times as the sending server tries to force its way past the grey-listing engine.

You can see GreyListing activity in the Email Activity reports, Mail Activity Log and the Transmission Log.