string Email = abc@abc.com
SmtpClient client = new SmtpClient(ServerIPAddress);
MailMessage message = new MailMessage(messenger@abc.com,Email.Trim());
message.Subject ="Hi this is Ayush Chaudhary";
client.Credentials = new System.Net.NetworkCredential("MailServerUserName","Pwd");
client.Port = System.Convert.ToInt32(PortNo);
client.Send(message);
By Default port number is 25
for sending mail to other domain you have to give your domain username or password.
Is this post is relevant for you................ This sample can send mail to any other domain and in domain.