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I have a Windows Server 2008 R2 server with the SMTP service enabled, which provides mail relay to another server for some applications.

Sometimes, a message comes in with a wrong "To:" address, consisting of only a single word without a SMTP suffix, i.e. someone instead of

When this happens, the server treats the message as being addressed to its own fake local domain, i.e. someone@smtp.server.fqdn, and drops it into its "Drop" folder.

There is, of course, no user named someone on the server, and that address is obviously invalid; and yet, the server still drops the message there, instead of sending a NDR because someone without any domain suffix is not a valid SMTP address.

How can I force this server to send a NDR when such a message is received?

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up vote 2 down vote accepted

It looks like the behaviour you describe is default for the SMTP service in IIS and can't be changed:

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Rather than add an additional domain in the SMTP configuration, you should just have editing the default domain and changed it to There's no reason why you need to leave the default domain as it is. You might even be able to delete it, just leaving the one you created.

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The server is handling relay for all Internet domains, and as such it doesn't have one or more destination domains, but only a global smart host. The defaul domain is there only because the server created it automatically and won't let me delete it. – Massimo Mar 29 '12 at 13:01
Ah, I see what you're getting at. It looks like the behaviour you describe is default for the SMTP service in IIS and can't be changed:… – Chris McKeown Mar 29 '12 at 13:29
Looks like you're right. If you post it as an answer (or edit this one), I'm going to accept it. – Massimo Mar 30 '12 at 11:57
Will do, thanks :-) – Chris McKeown Mar 30 '12 at 12:03

If the SMTP service is accepting recipient addresses without a domain part, it is doing so in violation of RFC 2822 Section 3.4.1. In this case, the server should not send an NDR but reject the RCPT TO: command right away due to a malformed recipient address.

You might consider opening a support case with Microsoft PSS on this to get it fixed.

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Are you sure this is a RFC violation? Because this is Windows Server 2008 R2, and as IIS's SMTP server component has been around for quite a while, I'd guess it has always behaved this way... – Massimo Mar 29 '12 at 13:07
@Massimo no, I am not. I've referred to the wrong specs - RCPT TO syntax is defined in 2821 and seems to allow specifying the local address part only, although frankly I cannot see any use for it and it has not been allowed in the original SMTP RFC 821. – the-wabbit Mar 29 '12 at 15:10

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