During each analysis, notifications are computed for each subscribed user. Then, asynchronously, these notifications are sent via email.
To set the delay between processing of the notification queue, set the sonar.notifications.delay property (in seconds) in SONAR_HOME/conf/sonar.properties. The server must be restarted for the new value to be taken into account.
Note that sometimes there could be some discrepancy between values displayed in the email and values displayed on the web interface. This happens if another analysis has been run between the sending of the email and your clicking on the link to the Issues drill-down in this email.
Who gets notifications
Only users who subscribe themselves will get notifications. With one exception is no admin functionality to proactively subscribe another user. If you believe a user should be receiving notifications, then it's time to practice the gentle art of persuasion.
Notifications will automatically (without user opt-in) be sent to users with Quality Profile Administration rights when built-in quality profiles are updated. These updates can only happen via an upgrade of the relevant analyzer. This type of notification is on by default, and can be toggled globally in Administration > General Settings > General > General.
To configure the email server, go to Administration > General Settings > Email.
Check also the Server base URL property at Administration > General Settings > General to make sure that links in those notification emails will redirect to the right SonarQube server URL.