The SMTP server requires a secure connection or the client was not authenticated.

System.Net.Mail.SmtpClient smtpClient = new SmtpClient(strSmtpServer, int.Parse(strPort));
		smtpClient.UseDefaultCredentials = false;

		System.Net.Mail.MailMessage message = new System.Net.Mail.MailMessage();
		System.Net.NetworkCredential basicCredential = new System.Net.NetworkCredential();
		try
		{
			
			basicCredential.UserName = strSender;
			basicCredential.Password = strSenderPass;

			System.Net.Mail.MailAddress fromAddress = new System.Net.Mail.MailAddress(strSender, "NameToDisplay");
			message.From = fromAddress;//here you can set address
			message.To.Add(strReceiver);//here you can add multiple to
			message.Subject = strSubject;//subject of email
			//message.CC.Add("cc@site.com");//ccing the same email to other email address
			//message.Bcc.Add(new MailAddress("bcc@site.com"));//here you can add bcc address

			message.IsBodyHtml = true;	//To determine email body is html or not
			message.Body = strMessage;
			smtpClient.EnableSsl = true;
			smtpClient.Send(message);
		}

		catch (Exception ex)
		{
			return false;
		}

		return true;