Hi!
Most of the properties on the installation settings node is available on the WAFRuntime.Installation object. In your case you get the DefaultFromEmail like this:
WAFRuntime.Installation.DefaultFromEmail;
Note that not all of the installation settings are available as a shortcut here, you need to fetch the installation node itself. Like this:
Installation inst = WAFRuntime.Engine.SystemSession.GetContent<Installation>();
Note that we don't have to specify the node id, since there is only one Installation node allowed in one installation.