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ExeConfigurationFileMap fileMap = new ExeConfigurationFileMap();
            fileMap.ExeConfigFilename = ConfigurationManager.AppSettings["TransportAgentConfig"];
            Configuration config = ConfigurationManager.OpenMappedExeConfiguration(fileMap, ConfigurationUserLevel.None);
            string currentValue = config.AppSettings["CustomHeader"]; //is inaccessible to its protection level (error )                    

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<configSections>
    <sectionGroup name="applicationSettings" type="System.Configuration.ApplicationSettingsGroup, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" >
      <section name="TransportAgent.Properties.Settings" type="System.Configuration.ClientSettingsSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
      <section name="AntiSpam.Settings" type="System.Configuration.ClientSettingsSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
    </sectionGroup>
    <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />

  </configSections>


  <applicationSettings>
    <TransportAgent.Properties.Settings>
      <setting name="MailboxAgent" serializeAs="String">
        <value>MailboxAgent</value>
      </setting>
      <setting name="CustomHeader" serializeAs="String">
        <value>X-Unverified-Sender</value>

.....
....                    

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if (!IsPostBack)
            {
                ExeConfigurationFileMap fileMap = new ExeConfigurationFileMap();
                fileMap.ExeConfigFilename = ConfigurationManager.AppSettings["TransportAgentConfig"];
                Configuration config = ConfigurationManager.OpenMappedExeConfiguration(fileMap, ConfigurationUserLevel.None);
                ClientSettingsSection section = (ClientSettingsSection)config.GetSection("userSettings/TransportAgent.Properties.Settings");
                string currentValue = section.Settings.Get("DeleteSpam").Value.ValueXml.InnerText;
                if (currentValue.Equals("False", StringComparison.CurrentCultureIgnoreCase))
                {
                    spamFilter.Checked = false;

                }
                else
                {
                    spamFilter.Checked = true;

                }