I am working in and ASP.Net framework 3.5 APP

I am sending en Email and I am using SmtpClient client = new SmtpClient(); to set the credencials. For that I am doing

client.Credentials = new System.Net.NetworkCredential( USERNAME, PASSWORD);

But I have problems to access from outside the server. I am trying to set the DOMAIN like this

client.Credentials = new System.Net.NetworkCredential(@"DOMAIN" + USERNAME, PASSWORD);

but @"DOMAIN" + USERNAME is translate it to "DOMAIN\" + USERNAME and it does not work.

Is there a way to set the Domain? or make @”DOMAIN” to not be replace with “DOMAIN” ?

Thanks