wanip-checker/arch_linux/wanip-checker.install

post_install() {
  echo "Configure files /etc/ssmtp/revaliases, /etc/ssmtp/ssmtp.conf and /etc/ssmtp/wanchecker.conf
after which run:
systemctl enable wanchecker.timer
systemctl start wanchecker.timer

See example SSMTP configuration files on https://github.com/Fincer/wanip-checker
"
  chown root.mail /etc/ssmtp/wanchecker.{sh,conf}
}

post_upgrade() {
  post_install
}