ping google.com

As you can see, this only returns one of IP addresses assigned to google.com. To get the full list of IP addresses associated with google.com, you need to use a different command line like nslookup.

Or better, use PowerShell, like this shown below:

[System.Net.Dns]::GetHostEntry('google.com').AddressList.IPAddressToString

It calls up .Net DNS library and use the function GetHostEntry to get the full list of IP address associated with the specified domain DNS.

Dig it a little deeper by performing a MX lookup to all these domains that we had problem sending emails to and found out that pretty much all of them have secureserver.net associated as their MX records. Meaning that emails sent to their inboxes have to go through and pass secureserver.net.

And one of the requests in order to get pass secureserver.net is that IP address of sender’s email server has to have a valid Reverse DNS entry associated. The email won’t get rejected right away if the sender’s IP address doesn’t meet this requirement. It will be delayed and put in the message queue first to give you more time to fix the issue. And if the problem is still not resolved before the message expires, an NDR email will be sent to sender.

That makes sense. We have recently changed ISP and had one new external IP address assigned to our mail server. And one thing I totally forgot is to check this Reverse DNS. Once I got my ISP to change it to what I wanted, emails started to flow to these domains again. Problem solved!

So if you relay all your emails to your ISP before they are sent out, you actually don’t need to worry about it because they all have the proper Reverse DNS entry set up to IP addresses that belong to them.

But if your email setup like ours that all emails are sent out directly from our email server, then you will have to make sure you have the proper Reverse DNS set up on the IP addresses that are responsible for sending emails. Here is the guideline from unblock.secureserver.net.

Verify that your rDNS contains a name that includes "mail", "SMTP", "relay", or "MX". For example: mail.example.com, smtp.example.com, or mx1.example.com.