I notice that some network administrators are now using greylisting on their IPv6 mail server as there is no other IPv6 anti-spam solutions in the market. They should be minded that that greylisting might not work as expected in a dual-stack configuration. Why not ? It is because if the v6 channel does not accept the mail, the sending side will fall back immediately to deliver the same mail over v4 channel. Unless the greylisting is also employed on IPv4, I do not see the effectiveness of greylisting on IPv6 only for a dual-stack configured mail server. I have no idea whether this is a weakness or not.
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