<system.net>
  <mailSettings>
    <smtp configSource="configsmtp.config" />
  </mailSettings>
</system.net>

<?xml version="1.0" encoding="utf-8" ?>
<smtp deliveryMethod="Network">
  <network host="127.0.0.1" port="25" defaultCredentials="false" password="" userName ="" />
</smtp>

@echo off
@REM Makes copies of all .default files without the .default extension, only if it doesn't already exist. Does the same recursively through all child folders.
for /r %%f in (*.default) do (
    if not exist "%%~pnf" (echo Copying %%~pnf.default to %%~pnf & copy "%%f" "%%~pnf" /y)
)
echo Done.

<Project>
   ...
    <Target Name="AfterBuild">
      <Copy SourceFiles="web.config" DestinationFiles="obj$(Configuration)tempweb.config" />
      <TransformXml Source="obj$(Configuration)tempweb.config"
                  Transform="web.$(USERNAME).config"
                  Destination="obj$(Configuration)tempweb2.config" />
      <ReadLinesFromFile File="obj$(Configuration)tempweb2.config"><Output TaskParameter="Lines" ItemName="TransformedWebConfig"/></ReadLinesFromFile>
      <ReadLinesFromFile File="web.config"><Output TaskParameter="Lines" ItemName="UnTransformedWebConfig"/></ReadLinesFromFile>
      <Copy Condition=" @(UnTransformedWebConfig) != @(TransformedWebConfig) " SourceFiles="obj$(Configuration)tempweb2.config" DestinationFiles="web.config" OverwriteReadOnlyFiles="True" />
    </Target>
  </Project>

<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
    <connectionStrings>
      <add name="MyDB"
        connectionString="Data Source=ReleaseSQLServer;Initial Catalog=MyReleaseDB;Integrated Security=True"
        xdt:Transform="SetAttributes" xdt:Locator="Match(name)" />
    </connectionStrings>
</configuration>

<configuration>
    <connectionStrings configSource="ConnectionStrings.config" />
...
</configuration>

<appSettings configSource="_configs/AppSettings.config" />

<appSettings file="C:/Standard_Path_To_Configs/Standard_Name_For_Config.config">