You enter the information here that is required to accept and deliver mail properly.
Even if email can come from different machines in your organization, you may want to hide this fact and present email as if it was from a single machine or domain.
This field is normally used in most setups and simply contains your official domain name.
If you are configuring the main server of an organization, you must check this field as "on." Normally, a sendmail server will accept only email sent to its fully qualified host name. If this is the main server, it will generally receive email addressed to the domain, not the server's name. Checking this box allows the server to accept both as equivalent.
For example, if your domain is foo.com and your server is mail.foo.com, then mail to joe@foo.com and mail to joe@mail.foo.com will be accepted and stored locally on this server (if user joe exists).
In most organizations, there is one machine which stores email for all users, even if the email originated from different machines. UNIX machines are multi-user. It is possible for a user to send email to other users on this machine. The default behavior is to store the message in the mail folder of the user on this machine. This is often inappropriate. UNIX machines are not only multi-user, but network oriented. This user (the recipient) may very well have accounts on many machines on the network (often on all machines) and may elect to read email messages on another machine. Having his email messages distributed onto different machines on the network is not useful.
Instead, specify the fully qualified name of the mail server of the organization, and all email messages will be forward to it.
Most machines on a network lack the connectivity to forward email properly. So, you can specify the name of a machine which knows better. In many organizations, this machine is often the mail server.
Unlike the mail server, the mail gateway may be reached by different
mailer transport/protocols such as ESMTP and UUCP.
You must specify how to reach the mail gateway. If you choose
ESMTP, then you must specify a fully qualified name
for the mail gateway.
If you specify UUCP, then you must specify a uucp name
for the mail gateway.
For on-demand Internet connections, the "expensive ESMTP" protocol is probably what you want. It queues all out-going mail. To deliver queued mail, you issue (using either scheduled tasks(cron) or the "process queue every (minutes)" field) a delivery command, which processes the queue.