Isobel,
Screen reading can be a very difficult thing to do and do correctly. JAWS is a pretty good program, that being said I have seen issues with Outlook and it not saying everything, and in my experience this is usually due to a new version of Outlook your version of JAWS does not support, or that you are using an old version of JAWS. Unfortunately I do not have the program myself to do testing on the deleting of an email.