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Emails Rejected by (reverse DNS failure?)

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Emails Rejected by (reverse DNS failure?)

Hello everyone,

   I am the System Administrator for a public library system.  We have our own exchange server that we use for all of our employees over several branches.  It seems as though a couple of weeks ago we started having issues sending emails to both and email addresses.  When I send an email, the person never receives the email, and we do not get a rejection message for several days after the email is sent.  I am going to put a sample email here and "xxxxx" out the personal side of things to protect the person's personal email but that shouldn't affect being able to determine what the problem is here.  I have seen another post on this site for something very similar but that post seemed to have been caused by a dynamic address being used for the email server.  We have public static IP addresses assigned through our router to specific private IP addresses.  I will post the rejection message below and hopefully someone will be able to help me resolve this issue.


From: Mail Delivery System []
Sent: Monday, January 1, 2018 7:13 PM
To: xxxxx
Subject: Undeliverable: Pay Stub Date = 12/29/17


Delivery has failed to these recipients or groups:
A problem occurred during the delivery of this message to this e-mail address. Try sending this message again. If the problem continues, please contact your helpdesk.

The following organization rejected your message:

Diagnostic information for administrators:

Generating server: #< #4.0.0 smtp; 421 Reverse DNS failure : Try again later> #SMTP#

Original message headers:

Return-Path: <>Received: from (localhost.localdomain [])  by localhost (Email Security Appliance) with SMTP id ACEC17F464_A4436C9B      for <>; Thu, 28 Dec 2017 00:11:53 +0000 (GMT)Received: from (unknown [])  by (Sophos Email Appliance) with ESMTP id 776D18251D_A4436C9F        for <>; Thu, 28 Dec 2017 00:11:53 +0000 (GMT)Received: from ([]) by ([]) with mapi id 14.03.0319.002; Wed, 27 Dec 2017 19:11:39 -0500From: Doug Workinger <>To: "" <>Subject: Fwd: Pay Stub Date = 12/29/17Thread-Topic: Pay Stub Date = 12/29/17Thread-Index: AdN/W5I3aXgtt4DTS9ODyYjPI2JdUQAFNwR8Date: Thu, 28 Dec 2017 00:11:38 +0000Message-ID: <>References: <>In-Reply-To: <>Accept-Language: en-USContent-Language: en-USX-MS-Has-Attach: yesX-MS-TNEF-Correlator:Content-Type: multipart/mixed;        boundary="_005_4348ABE1012341EEAF6749F68E450DDBkhcplorg_"MIME-Version: 1.0


Edit:  I can also message someone the static public IP address if it is absolutely needed.

Official Employee

Re: Emails Rejected by (reverse DNS failure?)


cyotie911, have you confirm the accuracy of these Comcast emails?