In User tasks there is the "Allow Email notifications" checkbox that you can use, and define the content of the email with the "Email template" field. When the task is re-assigned to other user I think the only option is a Task listener. If you find another simpler approach, please share it. Thanks.
I'm using APS and this is an option of the BPMN editor in activiti-app. If you have put a "User task" in the canvas, in the properties pane you will find the option to send the email. The Email templates are in the: Identity Management -> Tenants -> Email template.
I haven't worked with the Community Edition, so I'm not sure whether this is available there too; sorry.
Would you explain how are you implemented this using listeners? Yesterday I was trying this approach woth no success.
Unfortunately, in the community version, we do not have activity-app.
If you want to implement it using TaskListeners:
1. In your task properties add new TaskListener as Java class on event "complete".
2. Create a Java class and inside the execute function (don't forget about implements TaskListener ) call function similar to this one: (getServiceRegistry() is my custom function taken from ActivitiScriptBase.java)