Account -> Properties -> Templates -> Reading confirmation -> Action=Ignore

I presume that there's only a reading confirmation when the sender is asking for one. In case there's a reading confirmation for every message, my best guess is (as that isn't default behaviour for TB) that you created a read messages filter that is doing this.
In the latter case you can disable/delete/finetune that filter in:
Account -> Sorting Office/Filters -> Read messages

In both cases these are account specific settings. When the problem occurs in multiple accounts you'd better also check the read messages sector of the common filters.