Directive Needed:
using System.Web.Mail;

Code:
System.Web.Mail.MailMessage message = new System.Web.Mail.MailMessage();
message.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", 1);
message.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", "konrad12315");
message.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", "beatka1");
message.From = "[e-mail]";
message.To = "[e-mail]";
message.Subject = txtTresc.Text;
message.Body = rtbTresc.Text;
System.Web.Mail.SmtpMail.SmtpServer = "smtp.wp.pl";
System.Web.Mail.SmtpMail.Send(message);