Cannot send emails to foreign domains

I try to send emails through ASP.NET interface and can't get it to work. Emails are sent, but only if the receiver of the email has the same domain name as my s开发者_JAVA技巧erver (receiver@mydomain.com). The weird thing is, I actually CAN send such emails from any email program. So it seems that my Web.config file is missing something (?), although it looks ok (for me):

    <mailSettings>
        <smtp deliveryMethod="Network">
            <network host="mydomain.com" userName="info@mydomain.com" password="XXX" port="25" defaultCredentials="false" />    
        </smtp>
    </mailSettings>

Any ideas?

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OK, it was a provider issue