ASP Email Script Failing

Question

Im quite new to scripting, i have a support team that has a requirement for an app theyre developing and they need to send emails using the script they provided. Now they asked for a mailbo but i recommended simply relaying since we have an email gateway, similar to all systems i have internally. Usually i input only the email (******@mydomain.com), no password, specify my smtp server and port, no SSL and email goes through.

However with this configuration we are receiving an error within the logs " elay mailbox unavailable. the server response was no such domain at this location at system.net.mail.smtptransport.sendmail"

The script as follows;
<appsettings>
<add key="Username" value="******@mydomain.com" />
<add key="password" value="empty" />
<add key="smtpserver" value="smtp.mydomain.com" />
<add key="smtpPort" value="25" />
<add key="EnableSSL" value="false" />
<add key="Servicepollinterval" value="3000" />
</appsettings>

Hope this is not confusing, kindly assist.