Thanks AKI! I'm a step closer to achieving my goals.
However, I'm getting a new error as indicated from below:
Apr 11 01:45:34 lmtp(user@mydomain.com)<20801><pVlIEJ7wrlxBUQAAUdwEFQ>: Debug: push-notification-ox: Skipped because not active (/private/vendor/vendor.dovecot/pvt/server/vendor/vendor.dovecot/http-notify METADATA not set)
Apr 11 01:45:34 lmtp(user@mydomain.com)<20801><pVlIEJ7wrlxBUQAAUdwEFQ>: Debug: push-notification: Push notification transaction completed
By the way, the file /private/vendor/vendor.dovecot/pvt/server/vendor/vendor.dovecot/http-notify does not exist on my server.