Installation guide

Chapter 15. Email 215
In this situation, the sendmail server needs to masquerade the machine names on the company network
so that their return address is [e-mail] instead of [e-mail].
To do this, add the following lines to /etc/mail/sendmail.mc.
FEATURE(always_add_domain)dnl
FEATURE(‘masquerade_entire_domain’)
FEATURE(‘masquerade_envelope’)
FEATURE(‘allmasquerade’)
MASQUERADE_AS(‘bigcorp.com.’)
MASQUERADE_DOMAIN(‘bigcorp.com.’)
MASQUERADE_AS(bigcorp.com)
After generating a new sendmail.cf using m4, this configuration will make all mail from inside the
network appear as if it were sent from bigcorp.com.
15.3.5. Stopping Spam with Sendmail
Email spam can be defined as unnecessary and unwanted email received by a user who never requested
the communication. It is a disruptive, costly, and widespread abuse of Internet communication stan-
dards.
Sendmail has made it relatively easy to block new spamming techniques being employed to send
junk email using your system. It even blocks many of the more usual spamming methods by default.
You would need to consciously activate them by changing your /etc/mail/sendmail.mc 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 Sendmail version 8.9. 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 the /etc/mail/relay-domains file and restart Sendmail by typing the ser-
vice sendmail restart command as root to activate the changes.
However, many times, your users may be bombarded with spam from other servers throughout the In-
ternet that are beyond your control. In these instances, you can use Sendmail’s access control features
available through the /etc/mail/access file. As root, add the domains that you would like to block
or specifically allow access, such as in this example:
badspammer.com 550 Go away and do not spam us anymore
tux.badspammer.com OK
10.0 RELAY
Because /etc/mail/access.db 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 would be blocked with a 550
RFC-821 compliant error code and message back to the spammer, except for email sent 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.