Enquiry form error on ASP website

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ASP.NET

Hey guys,

I got roped into helping out a friend with an ASP website, I'm a php guy so I'm pulling my hair out! Since migrating the hosting for this asp website the online inquiry form has stopped working, I've updated the web.config settings to the new hosting details as below:

<appSettings>
<add key="sitelevel" value="2"/>
<add key="sign" value="$" />
<add key="siteurl" value="http://www.domain.com/" />
<add key="fromemail" value="sales@domain.com" />
<add key="mailusername" value="sales@domain.com" />
<add key="mailpassword" value="removed" />
<add key="mailserver" value="mail.domain.com" />
</appSettings>

Whenever someone submits the enquiry it just returns the error:

"Failure sending mail."

I've used telnet to connect to the mail server on port 25 and it seems to work just fine.

I'm bashing my head against a wall here so any advice on where to look would be much appreciated.