C#

static readonly string configFileName = "email.config";

private void saveBtn_Click(object sender, EventArgs e)
{
	try
	{
		// save everything and close
		string serverAddr = serverAddressTxtBox.Text;
		string port = portTxtBox.Text;
		bool ssl = sslRadioYes.Checked == true ? true : false;
		string senderAddr = senderTxtBox.Text;
		string password = passwordTxtBox.Text.Length != 0 ? Encrypt(passwordTxtBox.Text) : "";
		string subject = subjectTxtBox.Text;
		string attachment = attachTxtBox.Text;
		string messageBody = msgBodyTxtBox.Text;
		var configMap = new ExeConfigurationFileMap { ExeConfigFilename = configFileName };
		Configuration externalConfig = ConfigurationManager.OpenMappedExeConfiguration(configMap, ConfigurationUserLevel.None);
			
		/* If attachment points to a file outside the exes directory
		externalConfig doesn't reads the config properly
		and throws exception of type System.NullReferenceException */
		
		// here the exception is thrown
		externalConfig.AppSettings.Settings["ServerAddress"].Value = serverAddr;
		externalConfig.AppSettings.Settings["Port"].Value = port;
		externalConfig.AppSettings.Settings["SSL"].Value = ssl.ToString();
		externalConfig.AppSettings.Settings["Sender"].Value = senderAddr;
		externalConfig.AppSettings.Settings["SenderPassword"].Value = password;
		externalConfig.AppSettings.Settings["Subject"].Value = subject;
		externalConfig.AppSettings.Settings["AttachmentPath"].Value = attachment;
		externalConfig.AppSettings.Settings["Body"].Value = messageBody;

			// Save values in config
		externalConfig.Save(ConfigurationSaveMode.Full);
		Application.Exit();
	}
	catch (System.Exception ex)
	{
		MessageBox.Show("Error: " + ex.Message);
		Application.Exit();
	}
}