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Having a strange problem here.

We send out emails from a web application that use an SMTP server setup on a Windows 2008 R2 server.

Everything was working fine but one of our customers changed their incoming mail program (that's the best I could get out of their IT department) and now they're rejecting all our emails.

They did some digging and said that they're getting rejected because of non-alphanumeric characters in our originating server name.

Emails originating from our SMTP server come from They've told me it's the underscore that is doing it.

Now they asked us if we could change our domain name which of course isn't happening. Another solution would be to relay through another server but that's going to be a pain.

I was wondering, can I spoof the domain name so the originating server will be or something that doesn't have the underscore?

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Knowing what SMTP server you're running would be helpful. – joeqwerty Aug 18 '11 at 20:15
Sorry, IIS 6.0...SMTP Protocol 7.5. – Tom Aug 18 '11 at 20:19
up vote 5 down vote accepted

Go to IIS 6 Manager from Administrative Tools, then expand your server, then right click the SMTP Virtual Server and select properties, then click the Delivery tab, then click the Advanced button, in the "Fully-qualified domain name" field type in the name that you want the server to identify itself as.

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That worked, thanks. I'll test with the customer tomorrow and hope that a dash doesn't trip them up too. – Tom Aug 18 '11 at 20:32
Glad to help... – joeqwerty Aug 18 '11 at 20:34

Underscore is not a valid character in a host name, even though many programs will let you get away with it.


A "name" (Net, Host, Gateway, or Domain name) is a text string up to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus sign (-), and period (.).

I've had underscores in the host name cause problems in browsers before.

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I was able to take out the underscore domain name and just deliver as the server name which is MACHINE-NAME. Just tested it with the customer and it's still not getting through, seems like that dash is a problem too. – Tom Aug 18 '11 at 20:53

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