Greylisting is an important tool in the war against spam. Servers who connect to a mail server with an active greylisting are given a 450 recipent address rejected error the first time they try to connect and will not be allowed to successfully connect for a predefined time frame (normally a couple of minutes). A 450 SMTP error is not a permanent error and an RFC compliant mail server will try and resend the email again a bit later on. On the other hand the tools spammers tend to use do not adhere to standards and will not bother to retry sending the message again.
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