Home » Blog » Exchange Server » [Error Solved] Exchange Server Error 451 4.4.0 DNS Query Failed

[Error Solved] Exchange Server Error 451 4.4.0 DNS Query Failed

Ashwani Tiwari | Modified: 2022-01-11T10:37:49+05:30| Exchange Server | 5 Minutes Reading

Many customers have recently reported receiving Exchange Server Error 451 4.4.0 DNS Query Failed emails that are not being delivered to specific or other domains.  Although this problem occurs when the system’s DNS lookup fails due to a misconfiguration or because the recipient domain has explicitly blacklisted the address. Simple reconfiguration and connection testing performed to resolve this inquiry.
The Edge transport layer, which handles incoming and outbound email exchange, is tested using tools like NSLOOKUP and TELNET.

Symptoms That Point Towards The Exchange Server Error 451 4.4.0 DNS Query Failed:

  • Sending messages to a certain domain is not possible.
  • DNS query 451 4.4.0 Failed Error


  • Inbox and outbox mails become stuck or don’t get delivered, which causes problems for users.
  • Exchange 2007 Server versions with the Edge Transport Role installed frequently have this problem.
  • It has something to do with the edge transport layer’s mail queues.


If you have a firewall, make sure it allows traffic over ports 25 and 53 for SMTP and DNS resolution, respectively.

Also Read: What is Exchange Server Error Code 528 & How to Fix it?

Methods to Resolve DNS Query Failed Error 451 4.4.0

Configure the DNS Settings on the Edge Transport Layer Once More

Examining Queue Viewer:

  • Open EMC on the Edge Transport Server first.
  • Now select Toolbox>>Queue Viewer from the menu bar.
  • Search for Mail Flow Tools>> Queue Viewer.
  • Then, look in the column for the most recent error to see if any inbound email messages from any permitted domains are having the same issues.

Examining DNS Settings on the Edge Transport Server

  • Locally, you must log in to the Edge Transport server. If you’re using RDC 6.0 to connect to a physical server remotely, you should use the “/console” switch.
  • Now go to Exchange Management Console>>Edge Transport Server>>Properties and click OK.
  • Internal DNS lookups must select, and the default configuration must be set to All Available.

Performing DNS Lookups Both Internally and Externally

Internal Lookup:

Users who have several network adapters installed should go to the internal network card and now choose that card for DNS Network Card Settings.

  • This phase populates all possible IP addresses, which you can alter if there is any misconfiguration.
  • Restart Transport Service and check Exchange Management Console>>Edge Transport Server>>Properties to ensure the setup is correct.
  • If no IP addresses are detected, it’s possible that the NIC isn’t configure with DNS server entries. Now fill in the relevant information on the card, then double-check the transport server properties to ensure the configuration is correct.

External Lookup:

Modifying the settings of a single network card with public DNS may affect external name resolution and furthermore hinder the flow of mails. In this situation, you must select Use this DNS and an internal DNS IP address. Also, add a host file containing DNS information after that. Verify that the setup is proper in the final step.

Investigating DNS Servers and Name Resolutions

After we’ve checked the DNS servers’ setups, we need to see if they’re performing proper DNS name resolution or not. In this circumstance, 25 ports used to deliver SMTP/outbound emails, which is causing a queue problem due to incorrect DNS name resolution. To test the port, use NSLOOKUP.

Making Use of the NSLOOKUP Tool

NSLOOKUP is a command-line program for network administration that retrieves DNS records containing server and IP address information.

Locating the Server’s IP Address

  • To begin, open cmd and type “nslookup” followed by Enter. It will launch the NSLOOKUP utility.
  • After that, write set type=mx and press Enter.
  • After that, typeset timeout=20, because the default timeout for DNS queries is 15 seconds. Then, hit Enter after entering the domain name for which you want to remove the MX record.

Using TELNET to check SMTP Connection

  • Open a command prompt and type telnet.
  • Now typeset logfile (location:/filename) at the command prompt.
  • Then hit Enter after typing open mail1.google.com 25.
  • Type EHLO contoso.com and press Enter.
  • To create a text message, use options like MAIL FROM, RCPT TO, Notify, Data, and Subject.
  • Finally, the report prepared based on the configuration’s success. It could be a report on success or failure.

If the telnet services fail, check to see if the Windows Firewall enable or not. If it isn’t, set it up on NIC cards to allow services like SMTP, testing protocols like ICMP, and LDAP port access.

Professional Tool to Resolve the Exchange Server Error 451 4.4.0 DNS Query Failed

If the above-mentioned method fails to resolve the 451 4.4.0 DNS query failed Exchange Server error in the message queue, third-party software is always recommended. Exchange Recovery Tool is the best option for Exchange Server Error 451 4.4.0 DNS Query Failed troubleshooting. It is design with strong algorithms to repair Exchange Server mailbox items, assisting in the resolution of all Exchange Online Server issues. Furthermore, this application has a user-friendly interface that assures that the 451 4.4.0 DNS Query Failed error on the Exchange Server resolves without any technical issues.

Download for Windows Purchase Now


Microsoft Exchange Server is extensively used around the world, and it ensures that all of your business requirements are met. As a result of the Exchange Server error 451 4.4.0 DNS query failed issue, the user’s valuable work may be interrupted. Although the manual technique can be used, it necessitates technical expertise. If the manual way fails to resolve the problem, use the best expert method, as described above.