SMTP Permission Error

I'm using system.net.mail and have a textbox that users can enter their email address and a file gets attached and sent to them. When I test this in my custom box with Server 2008 I get the following error:

Request for the permission of type 'System.Net.Mail.SmtpPermission....at System.Security.CodeAccessSecurityEngine.Check

Do I have to configure something specifically on the server to allow? Or is it a code error?

string strto = txtTo.Text;  

//create the mail message
                MailMessage mail = new MailMessage();

                //set the addresses
                mail.From = new MailAddress("serveremail");
                mail.To.Add(strto);

                //set the content
                mail.Subject = "subject";


                //Get some binary data
                byte[] byteArray = Encoding.ASCII.GetBytes(result);


                //save the data to a memory stream
                using (MemoryStream ms = new MemoryStream(byteArray))

                    //create the attachment from a stream. Be sure to name the data with a file and 
                    //media type that is respective of the data
                    mai开发者_开发技巧l.Attachments.Add(new Attachment(ms, "test.txt", "text/plain"));

                //send the message
                SmtpClient smtp = new SmtpClient("server");
                smtp.Send(mail);

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This looks like it could be a permission issue when trying to access the attachment from the steam. What happens if you explicitly state credentials?