ASP.NET: Access Session variable in global.asax

It should work if you do it like this:

strError = System.Web.HttpContext.Current.Session["trCustomerEmail"]

Because that is what I do myself.

What exactly do you mean with: Visual Studio is telling that "Session is not available in this context"? Do you get a compiler error or a run-time exception?

You could try to be more defensive and test if there actually is a current HttpContext and a Session:

if (HttpContext.Current != null &&
    HttpContext.Current.Session != null) {
  strError = HttpContext.Current.Session["trCustomerEmail"]
}

I think applicaiton error is specific for the whole application and a session is specific for the user. Maybe you can throw your own Exception where you save the information from the session inside your exception.