The article is modified from Linux Configure Sendmail as SMTP Mail Client ( submission MTA )
In order to configure CentOS sendmail as a submission-only mail client, do follow the steps below. Sendmail can accept and send SMTP email requests from the local server. Outgoing MT always in a queue-only mode.
Configuring Sendmail in Queue-Only Mode
# vim /etc/sysconfig/sendmail
Modify the “DAEMON” line. Set DAEMON=no. This will make sendmail to be executed in a queue-only mode on the machine. The SMTP Server will sent but not receive mail requests.
Configure Mail Submission
Configure local server to use central MTA to be the sender of your mail for your domain
# service sendmail restart
$ mail -s 'Test Message' firstname.lastname@example.org < /dev/null