Unfortunately support does not answer even simple questions, like how to start validation.

At ZerIt, unfortunately, everything is hidden behind false labels.
Thereby verifying and finding emails is in itself a very simple process.

That's why it shouldn't be necessary to watch tutorial videos.

Since the disinterested support did not answer even my very simple question, I had to watch the tutorial videos and waste time.

The support answered only one question. So I learned that to upload existing addresses you have to start a campaign (but this action is not a campaign and therefore does not belong hidden there. Every useful software has an upload button for uploading lists).

The first tutorial video starts with the situation where all addresses are already loaded, which are also called "My Leads", although you initially only have addresses that might become leads some day.
That you get the addresses there by starting a campaign is not revealed there.

The interface and wording are completely wrong in all places.

I don't have the impression that the developers of ZeroIt really know what they are doing.

This is so critical because ZeroIt's extremely poor interface forces its customers to experiment to get the intended result (verify email).

Thus, the customer wastes time.

Another problem is that you can never be completely sure of anything

They obviously have no desire to provide support.
This matches the extremely low quality of the interface.
The results are then bound to be equally unreliable.