Exchange 2010 problem and fix: mail cannot be sent to a universal distribution list

Recently, I had to do some Exchange server problem solving, because a .NET application I had written could not send email to a universal distribution list configured in Active Directory. Briefly put, I had added a universal distribution list to AD, and then specified an email address for it: group@company.com.

Now, my .NET application simply uses regular framework SMTP classes to send email to this group, but the email never came to the inboxes of the people on the list. From code perspective, email was correctly sent without errors, but it never arrived.

Solving the issue took more than a couple of searches through Bing and Google, I wanted to document the solution here. When investigating the problem, I tested with Outlook to send email to the group. I received the following error message in the NDR report:

group@company.com
#< #5.1.1 smtp;550 5.1.1 RESOLVER.ADR.RecipNotFound; not found> #SMTP#

Thus, even though Exchange 2010 accepts the email for delivery (this means no errors are indicated to the sending application), it never finds the recipient list and reports the error 550, "RecipNotFound". What to do?

The answer is to modify the properties of the list in Exchange Management Console. Here are the steps:

1. Open Exchange Management Console and in the left-hand tree, open node for the server in question.
2. Go to Recipient Configuration and then to the Distribution Group tab.
3. Open the properties for the list that is giving you the hard time.
4. Go to the tab Mail Flow Settings, and select Message Delivery Restrictions. Click the Properties button.
5. In the dialog box that opens, uncheck the option "Require that all senders are authenticated".

After these steps, Exchange accepts non-authenticated SMTP email to your distribution list. Problem solved!