The original greylisting technology dictates to reject every message at the first attempt. In this way, regular messages are received by the client with some delay. This delay depends on the retry interval set on the sending server (from few seconds to few hours).
Greylite challenges a sending server for the first delivery. After the server proves able to pass the greylite challenge once, it is whitelisted.
Whitelisted address are pruned after 20 days (customizable) of inactivity.
This algorithm is modified by suspicion.