Email Setting with web config

In web Config add below section

<system.net>
  <mailSettings>
    <smtp deliveryMethod="Network" from="xxx@xxxxxxx.com">
       <network defaultCredentials="false" host="Smtp.gmail.com"
         password="xxxxxxx" port="25" userName="testing"/>
    </smtp>
  </mailSettings>
</system.net>

In Page write below code For Edit in web config.

using System.Configuration;
using System.Net.Configuration;
using System.Web.Configuration;

System.Configuration.Configuration config =
System.Web.Configuration.WebConfigurationManager.
OpenWebConfiguration(HttpContext.Current.Request.ApplicationPath);
System.Net.Configuration.MailSettingsSectionGroup settings
= (System.Net.Configuration.MailSettingsSectionGroup)
config.GetSectionGroup("system.net/mailSettings");
settings.Smtp.Network.UserName = txtIUsername.Text.Trim();
config.Save();