IP name possibly forged

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These days got a sendmail issue and fixed it after some research and tests, and I though it could be shared here.

The primary email server was and was used as relay server to send email to the internet, and I was configuring another server to send email through the relay server.

I added the network 192.168.6 to the access file and generated new access.db file, while when I did the test I got below error:

I made sure the email address xxxxxx@cn.ibm.com could receive email without any issue, and I did not know where the ip address 2XX.1XX.212.215 came.

I searched and found one reason was that the A record of the DNS server was different, while for me I did not use DNS to lookup both hostnames but the /etc/hosts files, and of course they matched on both servers.

At the beginning I lost minds in the sendmail configurations and wasted much time in finding the issues in the config files, while eventually I found the reason with really simple traceroute command:

As I checked this issue remotely and had no idea about the network configurations, so I did not think public ip address was used in the network path.

To fix this issue, I added the ip address 2XX.1XX.212.215 to the access file and then I would still get below warning:

While the email would be sent out. :)


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