Can you post your code over here,so that we can help you.
You can try this,
MailMessage mail = new MailMessage();
mail.To.Add("abc@gmail.com");
mail.To.Add("xyz@yahoo.com");
mail.From = new MailAddress("pqr@yahoo.com");
mail.Subject = "Send email";
string Body = "Here we are testing";
mail.Body = Body;
Attachment at = new Attachment("c:\\a.docx");
mail.Attachments.Add(at);
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient("localhost",25);
smtp.Host = "smtp.gmail.com";
smtp.Credentials = new System.Net.NetworkCredential
("abc@gmail.com", "mygmailpassword");
smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
smtp.EnableSsl = true;
smtp.Send(mail);
Label1.Text = "Mail has been sent";