I always do this...
[Unit]
Description=Postfix Mail Transport Agent
After=network.target
Conflicts=sendmail.service exim4.service
ConditionPathExists=/etc/postfix/main.cf
[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/bin/sh -c "for i in $(postmulti -l -a | awk '($3=="y") { print
$1}'); do systemctl start postfix@$i.service; done"
ExecReload=/usr/sbin/postfix quiet-reload
[Install]
WantedBy=multi-user.target
[Unit]
Description=Postfix Mail Transport Agent (instance %i)
PartOf=postfix.service
[Service]
Type=forking
GuessMainPID=no
ExecStartPre=/usr/lib/postfix/configure-instance.sh
ExecStart=/usr/sbin/postmulti -i %i -x /usr/sbin/postfix quiet-quick-start
ExecStop=/usr/sbin/postmulti -i %i -x /usr/sbin/postfix quiet-stop
[Install]
WantedBy=multi-user.target
