Exchange Server Error 451 4.4.0 DNS Query FailedBY EMAILDOCTOR.ORG

About 451 4.4.0 DNS Lookup Failed Error

• Now days many users are facing a common issue in email delivery from one domain to another domain.• User inbox & outbox mails get stuck or not delivered• Generally occur with installation of Edge Transport Role

in exchange server.• The error is related to Mail Queues & Edge Transport

Layer.

Edge Transport Server Role

Cause of Error & Symptoms• Cause of this error is generally related to

misconfiguration in system or the address manually blocked by the recipient domain user.• Users are unable to send Emails to any specific domain.

Prerequisites for easy resolve• First look for firewall and check whether it is allowing

transport at port 25 for SMTP and at port 53 for DNS resolution.

Inspecting Queue Viewer• Open Exchange management

Console on Edge Transport Server then go to Toolbox and then Queue Viewer.• Now in queue viewer locate mail

flow tools and the select queue viewer tool.

Verify DNS Configuration• User need to login locally in Edge Transport server and in

case of Remote desktop connection use Console switch.• Now open EMC & then Edge Transport Server and then

Properties.• Now select Internal DNS lookups & check for the default

configuration and it should be set to all.

Internal Lookup• In case of multiple network adapters, users need to go

to internal Network Card & select the card to access card DNS setting.• It will allow all IP addresses modification if any

misconfiguration occurs.• If there is not any IP address then NIC card might not

configured with DNS server entries. Fill in the card with accurate details and confirm the corrections.

External Lookup• This will work for single user who is having NIC and

using Public DNS, Changing the configuration might effect on external resolution and it might cause to stop email flow.

Testing DNS Server using NSLOOKUP & TELNET

• NSLOOKUP is a command line tool to get DNS records containing server information. • TELNET is also a command line tool to test ports.

Locate IP Address of Server• To Locate Server IP Address type “nslookup” and hit

enter to open the NSLOOKUP tool. • Now type set type = mx and hit enter• Now enter the domain name to extract the MX record

and press enter. For ex: bing.com

Test SMTP Connection using TELNET• Type telnet in cmd • Now type set logfile (location:/filename)• Now type open mail.abc.com 25 and hit enter• Now type EHLO contoso.com and hit enter• Use parameters like mail from, rcpt to, notify, data

and subject to compose a test email message.• Now at last the success or failure report will be

generated.

ConclusionWe discussed about the reason as well as the solution to the error “DNS query failed error 451 4.4.0”. As an exception case you can go to repair exchange mailbox and after successful repair exchange server mailbox can be accessed easily.

Thank You for watching