Once a Moodle forum post has been written, users have up to 30 minutes to edit their post, before it comes locked. If the `mail now¿ option is ticked, this 30 minute period is disabled and the post becomes locked immediately.
Email notifications from any forum posts (regardless of Moodle course) are sent to users hourly via an automated task on the Moodle server. This runs at 17 minutes past the hour. So, if the post was written at 9.10am and the `send immediately¿ option was ticked, the message would be sent to users at 9.17am. However, if it was written at 9.20am, it would be sent at 10.17am.
Users can opt to receive emails in two ways: when messages are posted to Moodle (within the hour) or as a digest of messages every 24 hours (at 5pm). These options are set up by each user in their Moodle profile. Because of this, a message posted at 9.17am in the example above might not be sent to the student until after 5pm (5.17pm) that same day.