Albinabel has given a right answer but it wil not work, because you need to specify the credentials i.e username, password and secure connection before sending the mail.
Following piece of code wil let u send email, and i have also provided it with google smtp server settings, so that you can check it easily
just replace network credentials with your google username, password, for to use with google,
Or to use with some other SMTP Server, change the host name also
MailAddress mafrom = new MailAddress("mani@abc.com");
MailAddress mato = new MailAddress("dontumindit@yahoo.com");
MailMessage mymsg = new MailMessage(mafrom, mato);
mymsg.Subject = "hy";
mymsg.Body = "How are You";
SmtpClient smptc = new SmtpClient("smtp.gmail.com", 587);
smptc.UseDefaultCredentials = false;
smptc.EnableSsl = true;
smptc.Credentials = new NetworkCredential("username", "password");
smptc.Send(mymsg);