add mail-cron systemd unit

etc/systemd/system/mail-cron.service

@@ -0,0 +1,8 @@
+[Unit]
+Description=Mail cron
+After=srv.mount
+Requires=srv.mount
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/php /srv/var/www/mail/cron.php

etc/systemd/system/mail-cron.timer

@@ -0,0 +1,14 @@
+[Unit]
+Description=Mail cron
+After=srv.mount
+Requires=srv.mount
+
+[Timer]
+# Time to wait after booting before we run first time
+OnBootSec=3min
+# Time between running each consecutive time
+OnUnitActiveSec=5m
+Unit=mail-cron.service
+
+[Install]
+WantedBy=multi-user.target