I don't want Postfix to confirm users via telnet

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I don't want Postfix to confirm users via telnet
\ Alejandro Facultad (15 Jan 2008)
. \ Matt Hayes (15 Jan 2008)
. . \ Alejandro Facultad (15 Jan 2008)
. . . \ Matt Hayes (15 Jan 2008)
. . . \ Noel Jones (15 Jan 2008)
. . . . \ Charles Marcus (15 Jan 2008)
. . . . . \ Noel Jones (15 Jan 2008)
. . . . . . \ Brian Collins (15 Jan 2008)
. \ mouss (15 Jan 2008)
. \ Geert Hendrickx (15 Jan 2008)
. \ Brian Evans (15 Jan 2008)

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Subject:	I don't want Postfix to confirm users via telnet
Group:	Postfix-users
From:	Alejandro Facultad
Date:	15 Jan 2008

Dear all, I have a Postfix mail server and when someone execute "telnet
mail.company.com.ar 25", after that I use my test account
to send a mesaage and test my server:

mail from: test <mailto:test>
250 2.1.0 Ok
rcpt to: invalid_user <mailto:aa>
550 5.1.1 <invalid_user <mailto:aa>>:
Recipient address rejected: User
unknown in virtual mailbox table
rcpt to: valid_user <mailto:mailvalido2>
250 2.1.5 Ok

Here I can see a hacker can put mail users in "RCPT TO"and verify they
exist. I don't want this because a hacker can make test several mail
users and after that he makes a list with valid users and use this list
for spam.

How can I setup Postfix to respond always "Invalid user" inclusive the
"rcpt to" mail user exist, if someone do a telnet like above ???

Thanks a lot

Alejandro