MSKB article# 289553

SYMPTOMS
When you attempt to send mail to an Exchange 2000 Server recipient or Exchange Server 2003 recipient, the message may not be delivered. The sender may receive a non-delivery report (NDR) from the system administrator similar to the following:
The following recipient(s) could not be reached:

User1@domain.com on 11/01/00 8:14 AM

The originator does not have permission to submit message
dns;Sendingdomain.com failed 5.7.1 smtp;550
5.7.1 Unable to relay for User1@domain.com

CAUSE
While there are several causes for this error, the most common one is that there may be no recipient policy for the expected domain in the Exchange 2000 organization or the Exchange 2003 organization.

RESOLUTION
Add a recipient policy for the expected domain. To do so:

1. In Exchange System Manager, expand the Recipients object, then expand the Recipient Policies object.

2. In the right-hand pane, right-click the Default Policy, and then click Properties. Click the E-Mail Addresses tab.

3. Click Add to add a new SMTP address.

4. In the SMTP Address Properties dialog box, in the Address field, enter
@expecteddomain.com
where @expecteddomain.com is the domain that then sender is attempting to send to. Click Apply, and then click OK.

5. In the Default Policy Properties dialog box, click the E-Mail Address tab, and then click to select the check box next to the SMTP address that you just created.

6. Click Apply. Click OK when you receive the following notification:
The e-mail Addresses of type(s) [smtp] have been modified. Do you want to update all corresponding recipient e-mail addresses to match these new address(es)?

NOTES: If you would like to make the new SMTP address the default reply-to address for all users in your Exchange 2000 organization, click the new address, click Set as Primary, click Apply, and then click OK.

If you want to update these addresses immediately, force the Recipient Update Service to update your Exchange server and organization.