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Communication Security: Remote Access and Messaging • Chapter 3 145

configuration files to restrict e-mail so that it has to originate or terminate at the
local server.The file /etc/sendmail.cf would be reconfigured to look like this:

Scheck_rcpt
# anything terminating locally is ok
R< $+ @ $=w > $@ OK
R< $+ @ $=R > $@ OK

# anything originating locally is ok
R$* $: $(dequote "" $&{client_name} $)
R$=w $@ OK
R$=R $@ OK
R$@ $@ OK

# anything else is bogus
R$* $#error $: "550 Relaying Denied"

The easiest way to prevent SMTP relay is to properly configure and test the
server during the build process. If you are not sure if your server is configured
properly, there is an easy way to test it.

NOTE

Do not try this from your internal network, since your server will allow
you to relay from the trusted internal domain.

1. Open a command prompt window (assuming you are using a Windows
OS running against Exchange).
2. At the command prompt, type Telnet <servername> 25.This will open
up a Telnet session with the SMTP server using SMTP port 25.
3. You will receive a response from the e-mail server giving the name of the
server, the e-mail software being used, and the date and time usually in the
format of 220 mail.fakeserver.com Microsoft ESMTP Mail Service/ Sendmail
8.11.6/SuSE Linux Mon, Oct 7, 2002 08:50:00.
4. Type HELO test.test.com.You will get a response from the e-mail
server, saying “Hello” back to you.

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