Intermittent email errors like 451 DNS temporary failure can be frustrating.

Mail work one day but can fail at another day. And, it makes the error fix quite tricky. The major causes of this 451 DNS error are incorrect DNS resolution, wrong nameserver settings and so on.

At Bobcares, we often receive requests to fix the 451 DNS temporary failure error as part of our Server Management Services.

Today, let’s get deep into this error and see how our Support Engineers fix it for customers.

What causes the 451 DNS temporary failure error?

Let’s begin by checking the details of the 451 error in detail.

Basically, it’s an email delivery error.

This error mainly occurs when the receiving SMTP server is not ready to accept the mail delivery. So, the sending SMTP server needs to retry the delivery at some later time.

A typical 451 bounce message when sending to a particular email address appears as:

Now, let’s get into a few reasons that cause this error.

1. Incorrect DNS resolution

Incorrect DNS resolution can cause this error to a greater extent. This can be due to temporary problems too. So initially our Support Engineers do a DNS lookup of the domain name to get the MX record.

After getting the MX record, we check whether the availability of the corresponding mail server.

We then check and confirm that all these are resolving correctly. And, in case of network or DNS configuration errors, we work towards fixing the error.

2. Wrong nameserver settings

Similarly, incorrect nameserver settings can also result in this error. By default, the contents in the /etc/resolv.conf file decides the DNS name servers of the server.

This file contains the information that is read by the resolver during DNS resolution. So, we check whether the nameservers in the /etc/resolv.conf file is reachable from our server or not.

3. Greylisting

Recently, one of our customers contacted us with a 451 DNS temporary failure error. He was using Outlook webmail services.

When he tried to send emails to a particular domain, he got this error. But, the error was intermittent. At times, emails went out after several retries or never went at all.

Unfortunately, the DNS resolution of the domain and nameserver settings were working fine. The domain was using Outlook servers as the MX record. Here, the greylisting of the sending IP address was causing the mail failure.

How we resolved it?

Our Support Engineers checked and found out that the receiving server had the greylisting feature to prevent spamming.

Greylisting enables the server to reject any emails from unrecognized senders. And unfortunately, the receiving server greylisted the sending server IP.

Therefore, the Outlook rate limited the emails sent from the greylisted IP. So, to make email working, we had to remove the sending IP from greylist.

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Conclusion

In short, the 451 DNS temporary failure error occurs mainly due to incorrect DNS resolution, wrong nameserver settings, greylisting and so on. In today’s writeup, we saw how our Support Engineers fix this error for our customers.