Email spam can be defined as unnecessary and
unwanted email received by a user that probably doesn't know the sender
and never requested the communication. It is a very disruptive, costly, and
widespread abuse of Internet communication standards.
Thankfully, Sendmail has made it (relatively)
easy to block new spamming techniques being employed to send junk
email. It even blocks many of the more usual spamming methods by
default, so that you would need to consciously activate them by changing your
/etc/mail/sendmail.cf file in a particular way to
make your system susceptible. For example, forwarding of SMTP messages,
also referred to as SMTP relaying, has been
disabled by default since the 8.9 version of
Sendmail. Before this change occurred,
Sendmail would direct your mail host (x.org)
to accept messages from one party (y.com) and send them to a different
party (z.net). Now, however, you have to specifically tell
Sendmail to permit a domain to relay mail
through your domain. Simply edit
/etc/mail/relay-domains and restart
Sendmail by typing the /sbin/service
sendmail restart command as root to activate the changes.
However, many times, your users may be bombarded by spam from other
servers throughout the Internet beyond your control. In these instances,
you can use Sendmail's access control
features available through the /etc/mail/access
file. As root, simply add the domains that you would like to block or
specifically allow access, such as:
badspammer.com 550 Go away and don't spam us anymore
tux.badspammer.com OK
10.0 RELAY |
Because /etc/mail/access is a database, you
need to use makemap to activate your changes
by recreating the database map. This is easily done by running the
makemap hash /etc/mail/access < /etc/mail/access
command as root.
This example shows that any email sent from badspammer.com to you would
be blocked with 550 RFC 821 compliant error code and message back to the
spammer, except for email send from the tux.badspammer.com sub-domain,
which would be accepted. The last line shows that any email sent from
the 10.0.*.* network can be relayed through your mail server.
As you might expect, this example only scratches the surface of what
Sendmail can do in terms of allowing or blocking access. See the
/usr/share/doc/sendmail/README.cf for more
detailed information and examples.
