How to configure sendmail as a gateway

This is a simple how to about how to configure sendmail to act as a mail
gateway.

This how to will only talk about 3 files: access, mailertable and
relay-domains.

File relay-domains NOT included in sendmail distribution (at least in my
CentOS distribution), you have to create it manually.
All files (at least in my CentOS) located in directory /etc/mail

Access file
First, we put our host and our network to be able to relay through our
server.
localhost.localdomain RELAY
localhost RELAY
127.0.0.1 RELAY
192.168.1 RELAY
192.168.2 RELAY

If you use GreetPause feature, you can put no waiting for local email.
GreetPause:localhost 0
GreetPause:192.168.1 0
GreetPause:192.168.2 0

If you using DNSBL, and want to skip one or two domains from DNSBL check,
you can use this
Connect:friendlydomain.com OK

Now for the big part.
Put all the domain that you want to relay BUT put it as REJECT. This way by
default all emails to that domain will be rejected unless we specify
otherwise - in this case OK.

indomino.net REJECT
blablabla.org REJECT
something.edu REJECT

user1@indomino.net OK
user2@indomino.net OK
user3@indomino.net OK

user1@blablabla.org OK
user2@blablabla.org OK

user1@something.com OK

Finish. For access file, you can use make -C /etc/mail to 'compile' it.

Mailertable file
Now put this entry in mailertable, so sendmail will know to where it will
route the emails.
indomino.net smtp:smtp.indomino.net
blablabla.org smtp:[192.168.1.25]
something.edu esmtp:[192.168.1.50]

Finish. Do make -C /etc/mail to 'compile' it.

Relay-domains file.
Most forgotten file, I think because it's not included in most
distribution. You have to create it manually. Simply as type touch
/etc/mail/relay-domains to create the file.

Now type all the domain that you want to relay.
indomino.net
blablabla.org
something.edu

Finish. Now restart the server.

Steps to stop SPAM

There is a good article about steps that we can use to prevent spam, you
can find the very good article in here
http://www.spamhaus.org/effective_filtering.html.

Personally, I use spamhaus as my primary DNSBL. It never let me down. Off
course there are some legitimate emails that being block by spamhaus, very
few, and that's not my problem. :).
I believe email administrators and ISP admin must protect they servers and
their network from spammers.

These is my configuration to stop spam:

1. DNSBL test in mta level.
I use sendmail as my mta server, and activated DNSBL in sendmail. Why put
DNSBL in mta level? Because it can stop a lot of spam before it transferred
to our mail server. But because it will stop few legitimate emails, I try
to put 'not so aggressive' DNSBL like sbl from spamhaus (actually I use
sbl-xbl.spamhaus.org).

2. Greetpause
Great new feature from sendmail. This way all emails (except if we put some
conditional) will be forced to wait for HELO or EHLO command from the mta
server. If the email doesn't bother to wait for HELO or EHLO command, the
we don't bother to receive, we simply kick it.

3. Mailscanner
Great software. Nothing more to say. You can download the latest version in http://www.mailscanner.info. In Mailscanner I activate these
features:

3.1. Spamassassin
A must. In new version of spamassassin there a script called sa-update to update builtin spamassassin rules, you can use cron job to make it run automatically every day.
You can add additional rules for spamassassin, most of them are in http://www.rulesemporium.org, additional rules can be automatically updated with smart cron job called rules_du_jour.

3.1.1. DCC + Razor
Inside the Spamassassin, I turn on both DCC and Razor. Increase the spam
score enough to make it a spam.

3.1.2. Bayes
Help a lot to stop more spam. I create two mail box, one for spam and one
for ham (non spam). Each day, a cron job will learn from each mail box, and
learn it as spam or ham.

3.2. Clamav (for check viruses)
Every mail server need antivirus. Clamav is the best there is, and free.

3.3. Another DNSBL test (to increase score)
This DNSBL actually will increase the spam score.

That's are my configurations. Hope that will help you to create your own
configuration for battle against spam.