I found in Kmail a menu entry "File -> Followup Reminder Messages". It opens a windows with a table in the middle, but the table is allways empty. How to work with this feature ? The help says ... "Follow Up Reminder Agent is an akonadi agent which follow up reminder mail"

Nobody can tell me, how it works ... ...

The table is empty, because you did not add a reminder so far.
First click with the right mouse button an a message in the list window, select the follow up reminder in the contect menu and add a date for the follow up.
Now you can manage this entry in the table opened with menu entry "File -> Followup Reminder Messages".

I play couple days with this function ... All i can see now is a new task for each tracked mail.
What happends when:

• an answer mail arrives before the date to follow up ?
• no answer mail is arrived until the date to follow up ?