Users selected option for Mail Save - Detecting in Extension Manager

I have an Extension Manager hook which handles sent mails and I need to be able to determine what the user has requested regarding saving the mail if the mail save preferences are set to ‘Always Prompt’. I cannot see anything that gets passed to the backend document (which is what the extension manager hook gets) to allow me to determine which option the user selected in response to the ‘Save this new message’ prompt. Any ideas?