Sendmail will work out of the box, and use the FQDN to send email from. You may want to make sure the hostname is set correctly using Webmin first.
For FreeBSD:
Add to /etc/rc.conf:
postfix_enable="YES" sendmail_enable="NO" sendmail_submit_enable="NO" sendmail_outbound_enable="NO" sendmail_msp_queue_enable="NO"
Add to /etc/periodic.conf:
daily_clean_hoststat_enable="NO" daily_status_mail_rejects_enable="NO" daily_status_include_submit_mailq="NO" daily_submit_queuerun="NO"