See this code it may help you for sending mail in c#.net
using System.Net.Mail;
MailMessage mail = new MailMessage();
mail.To.Add("YourMailId@gmail.com");
mail.From = new MailAddress("YourmailId@gmail.com");
mail.Subject = "Full Details of Your TextBoxs";
string Body = "UserName "+TextBox1.Text;
Body += "UserEmail "+TextBox2.Text;
Body += "Phone number "+TextBox3.Text;
mail.Body = Body;
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
string EmailId ="Your MailId";
string EmailPassword = "Your mail Password";
smtp.Credentials = new System.Net.NetworkCredential(EmailId, EmailPassword);
smtp.EnableSsl = true;
smtp.Send(mail);