I have an app.config file with a tag for an email address. I'd like to change the email address at runtime and have it saved to the config file. Any ideas on how to do this?

Here's my codel:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
        <sectionGroup name="userSettings" type="System.Configuration.UserSettingsGroup, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" >
            <section name="FlexTime.Properties.Settings" type="System.Configuration.ClientSettingsSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" allowExeDefinition="MachineToLocalUser" requirePermission="false" />
        </sectionGroup>
    </configSections>
  <connectionStrings>
    <add name="FlexTime.Properties.Settings.InCheckConnectionString"
      connectionString="Data Source=SVNYDBSQLDEV1;Initial Catalog=InCheck;Integrated Security=True"
      providerName="System.Data.SqlClient" />
  </connectionStrings>
  <appSettings>
        <add key="server" value="172.16.202.66" />
        <add key="supvemail" value="jweil@nyhomes.org" />
        <add key="title" value="FlexTime" />
        <add key="conString" value="Initial Catalog=InCheck;Data Source=SVNYDBSQLDEV1;Integrated Security=SSPI;" />
    </appSettings>
  <userSettings>
    <FlexTime.Properties.Settings>
      <setting name="supvemail" serializeAs="String">
        <value>jweil@nyhomes.org</value>
      </setting>
      <setting name="title" serializeAs="String">
        <value>FlexTime</value>
      </setting>
    </FlexTime.Properties.Settings>
  </userSettings>
</configuration>

The code in my form.cs is this:

ConfigurationSettings.AppSettings["supvemail"] = this.txtSupervisorEmail.Text.Trim();

The value isn't saved and reverts back to the old application setting once the application is relaunched.