Email delay and undeliverable: Exchange Server, Sonicwall, or DNS?

Posted on 2007-07-20
Last Modified: 2013-11-30
I recently changed ISPs and the new IP used as our Sonicwall gateway address (which does NAT) did not have the reverse DNS set up correctly. We were soon blocked by because of this. After we got unblocked we were soon blocked again. I'm not sure of the exact reason for being listed again, but one of the possibilities was that we had a compromised machine on our network that was a spambot.  I monitor network traffic and that did not seem to be the case to me.

Just for fun, I blocked all SMTP traffic from our LAN to the WAN only allowing traffic to two IPs...the SMTP server at our ISP and Blackberry redirection traffic.

Now folks are occasionally getting the following error:

Subject: Delivery Status Notification (Delay)
This is an automatically generated Delivery Status Notification.
Delivery to the following recipients has been delayed.

Then days later possibly this message with "Undeliverable" in the subject line:

Your message did not reach some or all of the intended recipients.
 Subject:      RE: Joseph's email
      Sent:      7/17/2007 5:01 PM
The following recipient(s) could not be reached: on 7/19/2007 5:14 PM
            Could not deliver the message in the time limit specified.  Please retry or contact your administrator.
            <MAIL.MYDOMAIN.ORG #4.4.7>  <------------ Not actual domain name

This is through Exchange Server 2003 with a virtual smtp server set up to use my ISP's SMTP server.

It may be worth noting that incoming mail routes  through our web host and we also use the service of for spam filtering. So when I do a reverse dns lookup everything looks fine except the following:

*ERROR* A record for does not point back to original IP (A record may be cached).

Any help is greatly appreciated.

Question by:GritsRGood
    LVL 104

    Accepted Solution

    Where did you configure the smart host? On the SMTP virtual server or on an SMTP Connector? If it was the first, do the second. IT is bad practise to put the smart host on the SMTP virtual server.

    If you are getting delays from your server, then the problem is getting the email from your server to the ISP. Does the ISP have a cluster of servers for their smart host? Many do. Have you allowed access to all of the cluster?


    Author Comment

    It's configured on the connector. I saw that my old connector was still there (I had kept both in during the conversion...). I deleted the connector to the old ISP. Could that have caused the problem? If there are two connectors listed does it "round robin"?

    I'm also checking the cluster angle. Great suggestion.
    LVL 104

    Expert Comment

    If both connectors were configure for the namespace as * and the cost as 1, then they would both be used. Your old ISP may not allow connections to their smart host from outside of their network.


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