Resolution: Won't Fix
The Email-ext Template Plugin is very useful for easy configuration of standard Email notifications. For example notifying the requestor or the culprits.
But one point is missing: Each Projects may have an individual list of users, which should receive build mails. So the "Email-ext Template" configuration in the project should have a way to define variables (e.g. $DEFAULT_RECIPIENTS), which then could be used in the template.
Without this enhancement you have to configure Email-ext with the explicit user list in addition to the Email-ext-Template. The results in duplicate mails for those users, who appear in both lists (e.g. because they have committed something). This is not good for acceptance. So instead of using the mail-ext-template, the same configuration will be made on every single project in mail-ext instead.