exim and local_part

[Giuseppe Sacco <gsacco@hsomail.com>]

Hi debian community,
I have sone problem using exim on my system.
I have this setup:
fetchmail to get mail,
exim to send mail via SMTP,
netscape/emacs/elm as mail reader of various users.

so everytime a send a message to any user, let say joe.user
exim correctly change the 'to:' field to "joe.user@my.domain.name"
but then it claims:

joe.user@my.domain.name:
  unknown local-part "joe.user" in domain "my.domain.name"

what does it means? How can I set a pattern to allow sending mail to
all users in my.domain.name?

Thanks,
Giuseppe