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I'm running a CentOS 4.2 email server and an Exchange 2003 box, but I'm having problems with one (or both). I didn't build the system, and my boss won't let me touch them in terms of upgrading or starting over (which I have been begging for since May). Anyway, I don't bother with the exchange server and go straight to the CentOS box for my email and have no problem with it. There are some users on the network that connect through the exchange box and they are having email issues. Many can get emails in from the accounts that we host on the CentOS box which are not part of the Exchange, but if they try to reply to those emails, they get an immediate "Undeliverable" failure. These same users have no trouble sending to anyone else, it's just emails that we host. Any ideas?

Another issue is very poor delivery times from incoming emails from other networks, sometimes upwards of 2 days, but not consistently. I don't know if these issues are related or not.

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Why do you have two, apparently separate, email servers on your network? – DanBig Feb 9 '11 at 18:23
The details of the NDR would help us immensely in helping you figure out what's going on. – joeqwerty Feb 9 '11 at 18:23
As for why there are two email servers, one is internal and one is an external. The exchange server is for our internal office alone. The external one hosts email for several different domains. And for NDR, are you asking for Non-Delivery Report or Network Data Representation? – noneoftheabve Feb 9 '11 at 18:36
Well seeing as this question is related to "undeliverable" emails, I'm asking for the details of the Non-Delivery Report (undeliverable message) that you're getting. – joeqwerty Feb 9 '11 at 18:40
From: System Administrator To: Subject: Undeliverable: Delivery Status Notification (Failure Your message did not reach some or all of the intended recipients. Subject: Sent: 2/9/11 11:45am The following recipient(s) cannot be reached: on 2/9/2011 11:45am The e-mail account does not exist at the organization this message was sent to. Check the e-mail address, or contact the recipient directly to find out the correct address. < #5.1.1> – noneoftheabve Feb 9 '11 at 18:45

The first thing that come to mind is the name resolution issue between Exchange and CENTOS. If you will be able to provide more information about how do you have email domains configured and where DNS servers are, I believe it would be a bit easier to troubleshoot.

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We run a DNS Server too. What would you like to know about the email domain config? – noneoftheabve Feb 9 '11 at 18:16

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