Global.asax emails not being triggered by specific page

I have a website that accepts payments via PayPal. Part of the PayPal process involves the PayPal server sending what is called an Instant Payment Notification (IPN) post to one of my web pages that is not normally seen by anyone. This IPN handler page is designed to process the IPN post and return data to the PayPal server for validation without any user involvement. The vast majority of our PayPal transactions are processed correctly but on some rare occasions a customer's transaction is aborted even though payment has actually been made, which is clearly a very undesirable situation.
The only indication of the source of these errors is that my ISP's IIS log shows an HTTP error code of 302 on my IPN handler page and a diversion to my generic error page (which no human being actually sees).  
My Global.asax file is programmed to send me an email containing information about any error on my site and this normally works very reliably. However, it is no ...