disabling certain users from receiving external e-mail messages

I'm running my DNS/Mail server (sendmail) on a Fedora Core 3 box. By
default every user I
have in my /etc/passwd can send/receive e-mail messages. Is it
possible to configure sendmail so that e-mail messages that do not
originate from localhost will bounce back?

To rephrase: I want to have some users to be able to receive messages
that are local, and only a specific set of users be able to receive
messages from any other hosts.

I want to do it in a way so that from the outside world these users do
not exist:

----- The following addresses had permanent fatal errors -----
<user@domain>
(reason: 550 5.1.1 <user@domain>... User unknown)

----- Transcript of session follows -----
.... while talking to domain.:
DATA
<<< 550 5.1.1 <user@domain>... User unknown
550 5.1.1 <user@domain>... User unknown
<<< 503 5.0.0 Need RCPT (recipient)


Thanks.

Paolo

.

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