First of all you can use Session, Request, Response, Server, etc.
If I understood you correctly mailutil.vb is a class.
1. Add to this class:
using System.Web;
2. In the method where you want to retrieve say Session["myPassedID"] get the session object:
System.Web.SessionState.HttpSessionState session = HttpContext.Current.Session;
string myID = session["myPassedID"];
Good luck