I have tried to decrease the number of mails sent by the system to my manager so I went to manage/manage configuration/email notifications and uncheck all the boxes in the MANAGER column except:
Status changes to 'Reported'
Status changes to 'Assigned'
Status changes to 'Resolved'

Now my manager has receive no mails at all. Interestingly he does not receive any mails even if only one of the checkboxes are unchecked, so it looks really binary: if everything is checked then he receives mail for everything, if not everything is checked then he receive no mails at all

forum link: http://www.mantisbt.org/forums/viewtopic.php?f=3&t=13172

Now I have two choice:
1, I set the personal preferences of my Manager, but then he wont receive a note about an issue even if it is assigned to him
2, I leave all the checkboxes on and then my manager will be flooded by tons of emails per day.