Where to find default texts for system generated emails

Hi folks,

my boss provided a new task for my SugarCRM 6.5.20 (Build 1001) CEproject. It's a while ago now I'd created a customlogic.php for Opportunity and Task for the following cases:
  1. An Opportunity gets assigned to an Account- or to one or several Profession-Team(s)
    • in this case an email notification is generated (by the opportunity's customlogic.php) and is sent to all members of all affected teams
    • in addition a new task is created for that opportunity
  2. The assigned team denies the Opportunity-Task
    • in this case an email notification is generated (by the task's customlogic.php) and is sent to the Client Manager (the assigned_to user) of the opportunity
So far so good, but there is another mail created (seems to be by default) when the Client Manager (assigned_to) field of the opportunity gets changed. And our customer wants that mail to have exactly the same content as the emails described above.

So here comes my challenge:

I've searched up and down through Sugar files, Google, StackOverflow and the already answered questions here but without getting the answer I was looking for: Thanam o'n dhoul (sorry for cursing), where have I to look for the default system generated email texts to reach my aim to adjust it in the way our customer wishs?

I really hope somebody can help me as this task is quite frustrating.

PS: Please don't hesitate to answer in German or English, just as you prefer ;)