I'm facing a network problem. I can't tell exactly what is happening. The situation is as below:

I'm using the SMTP services to send mail from an internal network. The mail sent to my company's email address can be received successfully. But when the mail is sent to external mail server, it can not be reached. My company is using a DNS server. The server is behind a proxy server. It seems that the SMTP services can't resolve the name for external mail server. How to configure the DNS server and local NT Server so that it can by pass the proxy and resolve the external email server??