Note: This article was originally published in 2012. Some steps, commands, or software versions may have changed. Check the current Exchange documentation for the latest information.

Exchange Server 2010 Edge Transport plays a critical role in email hygiene and routing for organizations that place a dedicated SMTP gateway in their perimeter network. One of the most common and frustrating errors administrators encounter after deploying an Edge Transport server is the 451 4.4.0 DNS query failed status message on queued mail. This guide walks through every major cause of this error and provides concrete troubleshooting steps to resolve it.

Understanding the 451 4.4.0 DNS Query Failed Error

When Exchange Edge Transport attempts to deliver a message, it must resolve the destination address through DNS. The 451 4.4.0 response code is a temporary failure indicating that the DNS lookup required to determine the next hop for the message did not succeed. Messages remain in the queue and Exchange retries delivery according to the retry interval configured on the queue.

The full error typically appears in the queue viewer as:

451 4.4.0 DNS query failed. The error was: DNS query failed with error ServerFailure

or variations such as:

451 4.4.0 DNS query failed. The error was: ErrorTimeout

This error can affect both inbound routing (Edge to Hub Transport) and outbound routing (Edge to the internet), depending on which Send Connector is experiencing the failure.

Common Causes

1. DNS Server Misconfiguration on the Edge Transport Server

The most frequent cause is that the Edge Transport server is not using the correct DNS servers. Exchange Edge Transport has its own DNS resolution settings that are independent of the network adapter configuration. Even if your network interface card (NIC) is configured with the correct DNS server addresses, the Exchange transport service may not be using them.

Exchange Edge Transport has two separate DNS configuration areas:

  • Internal DNS Lookups — used to resolve names for mail destined to the internal Exchange organization (Edge to Hub Transport).
  • External DNS Lookups — used to resolve MX records for mail destined to the internet.

By default, both are configured to “Use network card DNS settings.” However, this automatic detection does not always work reliably, particularly when the server has multiple NICs or when the DNS suffix search order creates ambiguity.

2. EdgeSync Synchronization Failures

When the Edge server is subscribed to the Exchange organization, EdgeSync replicates configuration data from Active Directory to the Edge Transport server’s local AD LDS (ADAM) instance. If EdgeSync has failed or the subscription has expired, the Edge Transport server may not have the correct routing information for the Hub Transport server.

3. Send Connector Smart Host Showing ”—”

After EdgeSync subscription, the automatically created Send Connector displays -- as the smart host value. This is normal behavior and does not indicate a problem. The -- means that Exchange will use the Hub Transport server address obtained from the EdgeSync subscription data. Do not attempt to manually change this value unless you are intentionally reconfiguring the connector.

4. Network and Firewall Issues

DNS queries from the Edge Transport server may be blocked by firewall rules, particularly if the Edge server sits in a DMZ. Both UDP and TCP port 53 must be allowed from the Edge Transport server to the configured DNS servers.

5. Multiple Network Adapters

If the Edge Transport server has both an internal and external NIC (a common DMZ configuration), the DNS settings on each adapter and the adapter bind order can cause unpredictable DNS resolution behavior. The transport service may attempt to use the wrong adapter’s DNS settings.

Step-by-Step Troubleshooting

Step 1: Verify DNS Resolution from the Edge Server

Open a command prompt or PowerShell session on the Edge Transport server and test basic DNS resolution:

# Test resolution of the Hub Transport server FQDN
nslookup hubserver.contoso.com

# Test resolution of an external MX record
nslookup -type=mx gmail.com

# Test using a specific DNS server
nslookup hubserver.contoso.com 10.0.0.1

If nslookup fails, the problem is at the operating system or network level, not Exchange-specific. Verify that the DNS server IP addresses configured on the NIC are correct and reachable.

Step 2: Check the Exchange Transport DNS Settings

Open the Exchange Management Console, navigate to Server Configuration > Hub Transport (or Edge Transport if managing locally), right-click the Edge server, and select Properties. Examine both the Internal DNS Lookups tab and the External DNS Lookups tab.

Alternatively, use the Exchange Management Shell:

# View the current transport server DNS configuration
Get-TransportServer -Identity "EdgeServer01" | Format-List Name, InternalDNS*, ExternalDNS*

If the settings show “Use network card DNS settings” and you are experiencing failures, explicitly configure the DNS server addresses:

# Set internal DNS to use a specific DNS server
Set-TransportServer -Identity "EdgeServer01" -InternalDNSServers 10.0.0.1

# Set external DNS to use a specific DNS server (can be different from internal)
Set-TransportServer -Identity "EdgeServer01" -ExternalDNSServers 8.8.8.8,8.8.4.4

Step 3: Test EdgeSync Synchronization

From the Hub Transport server (not the Edge server), run:

# Test the EdgeSync synchronization status
Test-EdgeSynchronization -FullCompareMode

# Check the EdgeSync status for a specific Edge server
Test-EdgeSynchronization -TargetServer "EdgeServer01"

A healthy result shows SyncStatus: Normal and recent LastSynchronizedUtc timestamps. If synchronization has failed, you may need to re-subscribe the Edge server:

# On the Edge server, create a new subscription file
New-EdgeSubscription -FileName "C:\EdgeSubscription.xml"

# Copy the XML file to the Hub Transport server, then import it
New-EdgeSubscription -FileData ([byte[]]$(Get-Content -Path "C:\EdgeSubscription.xml" -Encoding Byte -ReadCount 0)) -Site "Default-First-Site-Name"

# Force an immediate sync
Start-EdgeSynchronization

Step 4: Verify Send Connector Configuration

List all Send Connectors and their settings:

Get-SendConnector | Format-List Name, AddressSpaces, SmartHosts, DNSRoutingEnabled, SourceTransportServers

For the EdgeSync-generated connector routing to the Hub Transport, ensure DNSRoutingEnabled is set appropriately. If the connector is configured to use DNS routing but the Edge server cannot resolve the Hub Transport FQDN, mail will queue with the 451 error.

Step 5: Check the Queue and Retry

After making DNS configuration changes, retry the queued messages:

# View all queues with a status other than Ready
Get-Queue | Where-Object {$_.Status -ne "Ready"}

# Retry all messages in a specific queue
Retry-Queue -Identity "EdgeServer01\1"

# Force retry on all queues
Get-Queue | Retry-Queue

Step 6: Examine Transport Logs

The protocol logs and connectivity logs provide detailed information about DNS resolution attempts:

# Check the connectivity log location
Get-TransportServer -Identity "EdgeServer01" | Format-List ConnectivityLogPath

# Common default path
# C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\Logs\Connectivity

Open the most recent connectivity log file and search for DNS-related error entries. These logs show each DNS query the transport service makes and whether it succeeded or failed.

The Solution That Resolved the Original Issue

In the scenario described in the original version of this article, the resolution was straightforward: instead of relying on the automatic “Use network card DNS settings” option, the internal DNS server address was explicitly entered in the Edge Transport server properties under the Internal DNS Lookups tab.

The reason this fixed the issue was that the Edge server had multiple DNS servers configured on the network adapter, and the transport service was selecting a DNS server that could not resolve the Hub Transport server’s FQDN. By explicitly specifying the correct internal DNS server, the Edge Transport server consistently resolved the Hub Transport address and mail flowed normally.

Preventing Future DNS Issues on Edge Transport

  1. Always explicitly configure DNS servers in the Exchange transport settings rather than relying on automatic NIC detection.
  2. Use separate DNS servers for internal and external lookups when possible. Internal DNS should point to your Active Directory DNS server; external DNS can point to your ISP’s resolvers or a public DNS service.
  3. Monitor EdgeSync regularly. Set up alerts for EdgeSync failures so you catch subscription expiration before it impacts mail flow.
  4. Test after every change using Test-EdgeSynchronization, nslookup, and the queue viewer to confirm that mail is flowing.
  5. Document your DNS architecture including which DNS servers the Edge server should use for internal versus external resolution.