SQLgrey，利用 450 的方式擋 spam…
SQLgrey is a postfix policy service implementing a grey-listing policy.
而 grey-listing policy 是：
Grey-listing is a mechanism implemented at the MTA (mail transfer agent) level. The goal is to prevent most junk mails (spams and virus alike) to reach the users’ mailboxes.
When a mailbox receives an email from an unknown contact (IP), it is first rejected with a “try again later”-message (SMTP 450 error). It gets delayed until the sender tries again (the sender is required by the SMTP protocol to retry the delivery).
Most of the MTAs do retry between 6 and 30 minutes after the first attempt, some (usually mailing-list managers) can wait for as much as 4 hours.
On the second try (after a configurable delay) the grey-listing mechanism accepts the email.
Nowadays most spam is not sent out using RFC compliant MTAs; the spamming software will not try again later. Virus using their own SMTP engine are blocked too.
簡單來說，幫你送信的 SMTP server 在收到 450 (try again later) 之後，會等一段時間再送，而 spam 的軟體常常是「射後不理」，所以利用 450 可以擋住 spam。看起來會蠻有效的，也許可以玩看看？