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Undeliverable Message

Last modified: June 23, 2022
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Overview/Problem of Undeliverable Message

This article explains how to fix the 451 Greylist system administrator’s notice by sending an email to someone with the same domain name. ‘451 Greylisted, please try again in XXX seconds’ will display this status.

What is Greylisting?

Greylisting is one method for preventing and minimising spam. Email servers having the possibility to temporarily delay emails until posts validate the message’s authenticity. Most spammers had previously sent a spam message to a user, so the message was not resentful. However, greylisting has evolved slightly over time and is still useful in conjunction with other anti-spam strategies.

How to Fix — Adjusting your SMTP Authentication

In order to fix the greylized problem for mail customers, your customers must use SMTP authentication. In Microsoft Outlook 2003, 2007, and 2010, this is a relatively quick solution.

The following settings must be changed in your mailbox account:

  • Open the Control Panel
  • Click Mail (or Mail x32 in my example>Click Email Accounts>Click Change>Click More Settings
  • Select the Outgoing Server tab
  • Check “My outgoing server (SMTP) requires authentication.

You can modify your settings to “Log on with” if you have special credentials you need to utilise.

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