During each analysis, the notifications are computed for each user who has subscribed to notifications. Then, in an asynchronous way, these notifications are sent to users by emails.
To set the delay between processing of notification queue, set the
sonar.notifications.delay property (in seconds) in SONAR_HOME/conf/sonar.properties. Note that the server has to 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 you clicking on the link to the Issues drill-down in this email.
To be able to send notifications by email, go to Settings > General Settings > General > Email and configure the email settings.