Helo command rejected: need fully-qualified hostname when sending emails (504)

Simple tip this afternoon. You may have got the following error when sending emails through ASP.Net’s built in mail server before:

From: postmaster@YourWebserversName [mailto:postmaster@YourWebserversName]
Sent: 29 June 2012 15:50
To: sender@sendingdomainname.com
Subject: Delivery Status Notification (Failure)

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.


Reporting-MTA: dns;YourWebserversName
Received-From-MTA: dns;YourWebserversName
Arrival-Date: Fri, 29 Jun 2012 15:50:30 +0100

Final-Recipient: rfc822;recipient@recievingdomainname.com
Action: failed
Status: 5.5.0
Diagnostic-Code: smtp;504 <YourWebserversName>: Helo command rejected: need fully-qualified hostname

The fix is easy:

  1. Open IIS
  2. View the properties of you Default SMTP Virtual Server
  3. Go to the “Delivery” tab
  4. Click the “Advanced” button (in the bottom right corner)
  5. Under “Fully-qualified domain name” enter a domain name that points to the server
  6. Click Ok until you’re back to IIS
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