When creating a filter, please add a “Test” button. Yandex Mail has this feature and it’s extremely convenient.

This way I could immediately see, without leaving the filter editor, whether my rule actually matches emails. Yandex shows a list of messages the filter would apply to.

Right now, many filters don’t work immediately even after saving and checking “apply to existing messages”, so I have to wait and guess whether I configured it correctly. This makes setting up filters slow and frustrating.