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421 validating sender

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document.

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.

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Sep 17, 2016. To validate the sender mail servers for authenticity, many recipient servers look up for reverse pointers. An RDNS helps to verify that the sender is a valid and trusted one. Due to this RDNS security restriction, recipient servers may reject mails from senders without RDNS and bounce emails with the error. 
13-Jul-2018 13:16
I'm assuming that your domain is hosted on the Shared environment. In this case, you should contact the service provider i.e. Hostgator. Generally, such errors occur when Google detects some unusual rate of the mail to Gmail from a mail server IP address. It may be possible that the other domains hosted on the same. 
13-Jul-2018 13:22
The problem is that it will generally be very cryptic, like "550 Requested action not taken mailbox unavailable" or "421 Try again later". What does these numbers mean? First of all not any. It's a not very clever strategy to prevent spamming. You should contact your ISP and ask them to allow you as a certified sender. 
13-Jul-2018 13:26
Excessive amounts of data sent to server. This error is. This error message is issued if grey listing is enabled, and the sender, recipient and IP address triplet does not match an existing greylisting triplet. If the client tries to tell hMailServer the recipient address before the sender address, this error message is issued. 
13-Jul-2018 13:29

421 validating sender introduction

421 validating sender

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