A default sendmail.cf file will be installed in /etc. The default configuration should work for most SMTP-only sites. It will not work for UUCP sites; you will need to generate a new sendmail.cf if you need to use UUCP mail transfers.
To generate a sendmail.cf, install m4 and the sendmail sources. Read the README in /usr/src/mail/sendmail-8.6.9/cf for more details on creating sendmail configuration files. Also, O'Reilly publishes a good sendmail reference called ``sendmail'' by Bryan Costales.
One common sendmail configuration is to have a single machine act as a mail gateway for all the machines on your network. For instance, at Red Hat Software we have a machine redhat.com that does all our mail. On that machine we simply need to add the names of machines for which redhat.com will handle mail to /etc/sendmail.cw. Here is an example:
# sendmail.cw - include all aliases for your machine here. torgo.redhat.com poodle.redhat.com devel.redhat.com
Then on the other machines, torgo, poodle, and devel, we need to edit /etc/sendmail.cf to ``masquerade'' as redhat.com when sending mail, and to forward any local mail processing to redhat.com. Find the DH and DM lines in /etc/sendmail.cf and edit them thusly:
# who gets all local email traffic ($R has precedence for unqualified names) DHredhat.com # who I masquerade as (null for no masquerading) DMredhat.com
With this type of configuration, all mail sent will appear as if it were sent from redhat.com and any mail sent to torgo.redhat.com or the other hosts, will be delivered to redhat.com.