Error 550.5.7.1 Unable to Relay

There could be several reasons for the above error. I recently ran into a specific situation on an Exchange 2003 Server where the Exchange administrator had recently configured some security options. Some of the clients on the Internet were using POP3 to access their e-mail. People on the internal network (e.g. techgalxaxy.net) were able to successfully send and receive e-mails from internal clients. However, from external POP3 clients on the Internet, users were unable to send e-mails to other domains (e.g. intel.com). They were getting Error 550.5.7.1 Unable to relay for… error in their Outlook clients.

After running several tests and verifying that the third-party anti-spam software was not the culprit, I focused on the SMTP server. It turned out that the administrator had blocked the relaying for authenticated computers, which is allowed by default. As soon as that option was checked, the external clients were able to send out e-mails.

Here’s the procedure for configuring this option. This procedure assumes you only have the Default SMTP Virtual Server running. You need to configure the following option on the specific SMTP server that’s having the problem.

In Exchange System Manager, go to Servers, server_name, Protocols, SMTP, Default SMTP Virtual Server.
1. Right-click Default SMTP Virtual Server and select Properties.
2. Click the Relay button.
3. Ensure that the box Allow all computers which successfully authenticate to relay, regardless of the list above is checked. If not, check the box and click OK twice.
4. Restart your SMTP Virtual Server.

Checking the above option doesn’t open your SMTP server for relaying. If you have any doubts, use the following URL to verify that your SMTP server is not relaying.

http://www.abuse.net/relay.html

User your SMTP server’s FQDN to test relaying, e.g. ns1.techgalaxy.net. You should see about 17 relay tests with a report of status at the end.