//Found out how to send authenticated email via System.Web.Mail at http://SystemWebMail.com (fact 3.8)
if(this.UserName != null && this.Password != null)
{
em.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", "1"); //basic authentication
em.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", this.UserName); //set your username here
em.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", this.Password); //set your password here