The SMTP allows for temporary rejection of incoming messages. Greylisting is the technique to temporarily reject messages from unknown sender mail servers. A temporary rejection is designated with a 4xx error code that is recognized by all normal MTAs, which then proceed to retry delivery later.
Greylisting is based on the premise that spammers and spambots will not re-try their messages. Instead, they will move on to the next message and next address. This is effective since a re-try attempt means the message and state of the process must be stored inherently increasing the cost incurred by the spammer, but a standard component of any legitimate sender's server.
