Using Server.GetLastError().InnerException.ToString() in global.asax

I've not used the global.asax before and would like to utilise it for e-mail details of error messages when they occur. I have the following code for my global.asax file and I can see that the routine is being called as I have replace the current code with a redirect statement to check, however as soon as I add 'Server.GetLastError().InnerException.ToString()' it stops working.

The code I am using is:

imports System.Web
imports System.Web.Mail
imports System.Text
imports System.Web.SessionState
imports System.IO

Public Class MyGlobal
    Inherits System.Web.HttpApplication

    Sub Application_Start(Sender As Object, E As EventArgs)
        ' Code that runs on application startup
  &nbs ...