A common misconception about email delivery is that it is instantly completed. Email is designed to be sent from Mail Server to Mail Server until it reaches the ultimate destination with no guarantee on how long it will reside at each intermediary stop. Most cases of delayed email are due to slow or overloaded mail servers (this includes but not limited to Google and AWS). Email delivery delays are caused by spam, frequency of checking one's email, or other hardware issues that either directly or indirectly effects the user.
In addition, students can update their Forum, Message, and Notification preferences so that they don't receive emails from Moodle.
We recommend that for students to keep up to date on course news they are reviewing their Moodle course on a consistent basis. One of the benefits of using the Announcement forum is that it will post the information in three places: in the announcement forum itself, in an email, and in the latest news block allowing multiple opportunities for students to review the information.