Why does my SMTP not work in Medium Trust Level?

Im Using asp.net 3.5. My SMTP Email works in Full trust though just not in Medium trust level.

So my SMTP works in Full trust, however when i change the trust level to "Medium" I get this error:
Exception Details: System.Security.SecurityException:
Request for the permission of type 'System.Net.Mail.SmtpPermission,
System, Version=2.0.0.0, Culture=neutral,
PublicKeyToken=b77a5c561934e089' failed.

public void SendEmail()
    {
        String EmailUser = ConfigurationManager.AppSettings["EmailUser"];
        String EmailPW = ConfigurationManager.AppSettings["EmailPassword"];

        MailMessage mail = new MailMessage("test@gmail.com", txbEmail.Text.Trim());
   &n ...