When setting up maintenance periods (recurrent ones are more affected) sometime the action get "stuck" and start sending thousands of email to the specified users for the action. This can happen with the default or the recovery message. It seems to be related to zabbix server not updating it's configuration when users add maintenance period. Here's a way to reproduce it (works most of the time) :

1 - Create items, triggers and actions for a host
2 - Activate one of the triggers/action that send a mail (you should receive an email activated by the trigger)
3 - Enter a maintenance period from now till some time in the future while the trigger is still active.
4 - Wait for 2 minutes... then try to toggle the trigger (on/off).

You can also try to configure a weekly maintenance, you should experience the problem. The only way I found to stop this from happening is to restart the zabbix server after setting up the maintenance periods. However not everyone has the rights to restart the server, so this isn't a very good workaround. You can see a thread about this in the forums :

http://www.zabbix.com/forum/showthread.php?t=15899&page=2

Thanks