This might be the behavior you want but if it is you should document it in the config file. Currently I think most people reading the config setting would assume that to make email notifications work all you need to do is turn them on, which is not the case. You also need to lower the value of threshold_min before any emails are sent.

This has been the default since mail notifications was introduced. There are notes in the manual on various useful settings (http://www.mantisbt.org/manual/manual.customizing.mantis.email.notifications.php).

Wouldn't having the min and max set to NOBODY = no emails being sent?
No, the thresholds are for ALL users with a certain access level

 * The following two config options allow you to control who should get email
 * notifications on different actions/statuses.  The first option (default_notify_flags)
 * sets the default values for different user categories.  The user categories
 * are:
 *
 *      'reporter': the reporter of the bug
 *       'handler': the handler of the bug
 *       'monitor': users who are monitoring a bug
 *      'bugnotes': users who have added a bugnote to the bug
 *      'explicit': users who are explicitly specified by the code based on the action (e.g. user added to monitor list).
 * 'threshold_max': all users with access <= max
 * 'threshold_min': ..and with access >= min