Email failed to deliver to certain domains with an SMTP protocol error occurred message

It happened lately in my office that emails sent to certain domains first got delayed and then failed with a SMTP error code 4.4.7 in the final NDR. Error code 4.4.7 usually indicates that the message in the queue has expired and usually the problem is on the receiving server. However, that is obviously not the case this time. Also on the server side, all queued messages all have one same warning message, “an SMTP protocol error occured”.

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 associated as their MX records. Meaning that emails sent to their inboxes have to go through and pass

And one of the requests in order to get pass 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

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

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