Greylisting

Greylisting is the technique first described by Evan Harris, of delaying mail from previously unseen "tuples" (originating IP, envelope-from, envelope-to) for a short initial retry period

Forces the sender to be RFC compliant and actually retry the message

Server remembers tuples and connect times as a "greylist entry". Once a host successfully retries after the initial delay period (typically 30 minutes) that host can send mail for a long while (usually 30 days or so)

If a host doesn't retry within a short period (usually 4 hours) the greylist entry is removed.