[Exim] can send but not receive

Hello All Exim Users,
I write from Italy and I've configured a Debian Server in my home on the
static IP of my ADSL connection.
I've installed Exim through apt-get and its installation script, setting it
with direct smtp connection. I can successfully send mail to anyone, but I
can only receive them in my local home network. Any attempt to send a mail to
my server from the Internet result in this error message: 5.1.1 not a valid
mail-box.

In the documentation I don't find anything useful to solve the problem. Can
you help me please?

Best wishes
Paolo