<system.net>
	<mailSettings>
		<smtp deliveryMethod="SpecifiedPickupDirectory">
			<specifiedPickupDirectory pickupDirectoryLocation="c:Tempmail"/>
		</smtp>
	</mailSettings>
</system.net>

<system.net>

    <mailSettings>

      <smtp deliveryMethod="Network">

        <network host="mail.mydomain.com" port="25" />

      </smtp>

      <!-- Use this setting for development

      <smtp deliveryMethod="SpecifiedPickupDirectory">

        <specifiedPickupDirectory pickupDirectoryLocation="C:Temp" />

      </smtp>
  -->

</mailSettings>

SmtpClient client = new SmtpClient();
    ...

    // Add "~" support for pickupdirectories.
    if (client.DeliveryMethod == SmtpDeliveryMethod.SpecifiedPickupDirectory && client.PickupDirectoryLocation.StartsWith("~"))
    {
        string root = AppDomain.CurrentDomain.BaseDirectory;
        string pickupRoot = client.PickupDirectoryLocation.Replace("~/", root);
        pickupRoot = pickupRoot.Replace("/",@"");
        client.PickupDirectoryLocation = pickupRoot;
    }

// Arrange - get SitePath from AppDomain.Current.BaseDirectory + ..
    string pickupPath = Path.Combine(SitePath, "App_Data", "TempSmtp");
    if (!Directory.Exists(pickupPath))
        Directory.CreateDirectory(pickupPath);

    foreach (string file in Directory.GetFiles(pickupPath, "*.eml"))
    {
        File.Delete(file);
    }

    // Act (send some emails)

    // Assert
    Assert.That(Directory.GetFiles(pickupPath, "*.eml").Count(), Is.EqualTo(1));

<system.net>
  <mailSettings>
    <smtp deliveryMethod="SpecifiedPickupDirectory">
        <specifiedPickupDirectory pickupDirectoryLocation="c:Tempmail"/>
        <network host="localhost" />
    </smtp>
  </mailSettings>
</system.net>