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 spamhaus.org 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.
THIS IS A WARNING MESSAGE ONLY.
YOU DO NOT NEED TO RESEND YOUR MESSAGE.
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:
email@example.com 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 MXLogic.com for spam filtering. So when I do a reverse dns lookup everything looks fine except the following:
*ERROR* A record for mydomain.org. does not point back to original IP (A record may be cached).
Any help is greatly appreciated.