How to resolve unable to relay error from web server smtp

My client's web application is throwing an error when trying to send an email:

Mailbox unavailable. The server response was 5.7.1 Unable to relay for

I don't believe this web server is available outside of my client's network. I believe they access the web site via a network path, something like http://central/MyWebApp/SomeFolder/SomePage.aspx.

Emails to local email addresses are able to go through.

1. If the web server application isn't available outside of the company's network, can the web server's smtp service still send to email addresses outside of the company?
2. Assuming that smtp should be able to send out this email, how can this be resolved? I am not a network person and my client's network tech doesn't seem to have the necessary skills. Is this an issue with their SMTP configuration? Do they need to contact their ISP? Do they need someone more knowledgeable?

Thank you for your assistance.
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The ISP may be preventing mail from being relayed through an outgoing mail server that's not their own -- they may want to control what mail is going out (to help mitigate malware, for example).

If you can find out what your ISP's SMTP server is, that may be a good first place to start. You may have to use that instead of the one you've been using.

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Two ways to handle this that I can think of.

1.  Have the SMTP server relay for the web server's IP address.  What this will do is any e-mail sent from the IP address of the server will be allowed to go outside of the company.

2.  Have your ASP.NET application authorize using a username and password for an existing (or new) e-mail account.

Since the web server is internal, my vote would be for #1.  If you need to go with #2, check out
ZekeLAAuthor Commented:
Thank you for your quick responses. I'll forward them to the network contact.
If the message is being generated by the internal exchange server, you may need to add the webapp server to the snmp connector groups so that it can have the mail relayed externally.
ZekeLAAuthor Commented:
The network person enabled relay which resolved the problem. Thanks for the help.
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