Restart=always
RestarSec=5
Then you need to reload the daemon configurations to ensure systemd is aware of this change by doing:
systemctl daemon-reload
systemctl restart postfix.service
server:~ # ps aux | grep postfix root 24446 0.0 0.0 34180 3216 ? Ss 12:53 0:00 /usr/lib/postfix/bin//master -w postfix 24447 0.0 0.1 35820 3996 ? S 12:53 0:00 pickup -l -t fifo -u postfix 24448 0.0 0.1 36204 5732 ? S 12:53 0:00 qmgr -l -t fifo -u root 24523 0.0 0.0 9288 1636 pts/0 S+ 13:08 0:00 grep --color=auto postfix server:~ # kill -9 24446 server:~ # ps aux | grep postfix root 24549 0.0 0.0 9288 1636 pts/0 S+ 13:08 0:00 grep --color=auto postfix server:~ # ps aux | grep postfix root 24644 0.0 0.0 34180 3216 ? Ss 13:08 0:00 /usr/lib/postfix/bin//master -w postfix 24645 0.0 0.1 35820 3992 ? S 13:08 0:00 pickup -l -t fifo -u postfix 24646 0.0 0.1 36204 5732 ? S 13:08 0:00 qmgr -l -t fifo -u root 24662 0.0 0.0 9288 1632 pts/0 S+ 13:08 0:00 grep --color=auto postfix server:~ #