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How to Fix Exchange Online Error 550 5.4.300 and 451 4.1.8 DNS Failure
Question details
Users are unable to send emails due to delivery failures, receiving bounce-back messages containing error codes 550 5.4.300 and 451 4.1.8.
- Product
- Exchange Online
- Device & OS
- not provided
- Scenario
- Sending emails to external recipients through Exchange Online or Outlook on the web.
- Observed behavior
- Mail delivery fails and returns Non-Delivery Reports (NDRs) indicating message expiration (550 5.4.300) and that the recipient domain does not resolve (451 4.1.8).
Verify that you have typed the recipient's email address and domain correctly, and prepare to use a command-line tool to check public DNS records.
Verify Recipient DNS Configuration Using Nslookup
Use the nslookup tool to confirm if the recipient's domain has valid Mail Exchanger (MX) records, which is the most common cause of the 451 4.1.8 error.
The 451 4.1.8 error indicates a DNS resolution problem on the recipient's end. When Exchange cannot resolve the domain, it keeps trying until the message eventually times out, triggering the 550 5.4.300 message expired error.
Press the Windows key, type 'cmd' into the search bar, and press Enter to open the Command Prompt.
Type 'nslookup -type=mx recipientdomain.com' (replace 'recipientdomain.com' with the actual domain of the email address you are trying to reach) and press Enter.
Review the output for valid MX records. If the command returns an error or no records, the recipient's DNS is misconfigured.
Reach out to the recipient via an alternative method (such as a phone call) and advise them to contact their IT administrator to fix their domain's DNS records.
Check Outbound Mail Routing and Relay Settings
Investigate your organization's outbound email routing if you have confirmed that the recipient's DNS is functioning perfectly.
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Frequently Asked Questions
What does Exchange Error 550 5.4.300 mean?
This error signifies that an email message expired because the Exchange server took too long to deliver it. It typically occurs after the server makes repeated failed delivery attempts over a specific period, usually due to unreachable destination servers or DNS issues.
How long does a 4xx temporary error delay my emails before failing permanently?
Exchange Online typically continues trying to deliver messages that encounter 4xx temporary errors for up to 48 hours. If the delivery issue on the recipient's end is not resolved within this timeframe, the system stops trying and generates a permanent 5xx Non-Delivery Report (NDR).
Can I fix Error 451 4.1.8 from my own Exchange admin center?
Generally, no. Error 451 4.1.8 specifically indicates a DNS resolution failure on the recipient's end. You will need to contact the recipient organization so their IT administrator can fix their domain's DNS or MX records.
