Undeliverable Message

Last modified: February 24, 2020
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Undeliverable Message and 451 Greylisted


This article discusses how you can send an email to someone with the same domain name to solve the message that the 451 Greylist system administrator receives.

system administrator with a undeliverable message regarding ‘451 Greylisted, please try again in XXX seconds’

What is Greylisting?

Greylisting is one way of stopping / reducing spam. Mail servers with the option to delay emails temporarily until posts verify that the message is genuine. In the previous days, most spammers once sent a spam message to a user, so the message wasn’t resentful. However, the greylisting has altered slightly today, and is thus still helpful today with other anti-spame techniques.

How to Fix – Adjusting your SMTP Authentication

Your customers must use SMTP authentication in order to fix the greylized error for the mail customers. This is a relatively fast solution in Microsoft Outlook 2003, 2007 and 2010.

Your mailbox account configurations need to be adjusted:

  • Open 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.”

If you have specific credentials you need to use, you can change your settings to “Log on using”

Send a test email now to someone with the identical domain name!


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