I was two weeks on holiday and just returned yesterday back to Germany.
I just installed Postnuke 0.723 with LAMPP 0.9.9a without any special configuration. I send me an email by Administration and Mail users. It worked perfectly.
Did you set an valid email address for your Postnuke admin? Administration -> Settings -> Administrator e-mail:. By default there is none@none.com set and you must(!) enter an valid email address. Otherwise it won't work. In my installation it doesn't work before I set this to an real address.