Built-in SMTP-server in the IIS returns 5.7.1 Unable to relay

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Built-in SMTP-server in the IIS returns "550 5.7.1 Unable to relay" after I had upgraded the server to Windows Server 2008 SP2...

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Built-in SMTP-server in the IIS returns 5.7.1 Unable to relay

I did a small upgrade of a server that I’m in control of. Well it was actually a rather big upgrade, I added the SP2 of a Windows Server 2008 Web Server. Everything went perfectly, until we noticed that the SMTP-server had stopped working.

The only response was “SMTP server response: 550 5.7.1 Unable to relay” in the Eventlog. Strange I thought, since it worked perfectly a moment ago!

I started my quest for getting this to work again and found the very simple solution, but yet too close for me to see. The only thing I had to do was to allow the SMTP-server to relay messages from the local computer.

How to allow Relay from the local machine in the built-in SMTP-server

All this relates to the built-in SMTP-server for the IIS, to see how to configure the SMTP-server visit the IIS.NET site.

First step is to open the IIS manager, that you can find under “Control Panel / Administrative Tools”.

Once you have the manager in front of you, select the SMTP-server node. In our case named “[SMTP Virtual Server #1]”.


Right-click and select Properties and you will get the following window.


Under the headline “Relay restrictions” click on “Relay…”. In the window that pops-up fill out the parameters as shown. What you want to do is to allow the Local Computer ( to be able to relay messages through this server.


That’s it, now it should be nice and dandy and you are ready to start sending a lot of email messages again!