Moving to postfix from sendmail is wonderful. Here’s how to set up a backup mx for a remote server. If you have many backup mx’s to enter, use a new line and put whitespace in front of the domain. See the instructions in /etc/postfix/main.cf for more details. Configure your dns with the appropriate mx records and then edit:
relay_domains = $mydestination thehenrys.net doctorslabinc.com pikecreekturf.com
smtpd_recipient_restrictions = permit_mynetworks, check_relay_domains
Restart postfix with:
/sbin/service postfix restart
Go to the park and get some fresh air.