421 validating sender
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Since many MTA clients see a 421 as a possible loading limit, it may use this to immediately reschedule a retry using a different MX/IP host..The high payoff has been the nearly 100% of all mail senders behaving as "single shot" mail senders are abusive and/or malicious in nature.Greylisting can not address abusive mail senders using compliant SMTP mail clients.Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.mail systems to limit the amount of unwanted, abusive mail that they receive and deliver to their users.
It was designed to be scalable and marginal impact to both administrators and users, and should be acceptable for use on a wide range of systems.