Especially about the future.
(Quote Investigator <
http://quoteinvestigator.com/2013/10/20/no-predict/>)
However, I think in _The Wizard of Oz_, the Wicked Witch's
crystal ball only worked like television. To view at a
distance.
What's going on in that part of the story is that the
wizard world is saying to the Dursleys: You can stop
Harry from getting our letter, but you can't stop us
/sending/ the letter. And you can't stop us knowing
where Harry is. Then the climax is when Hagrid
turns up.
In-story, it's a situation where wizard laws about
limiting contact with Muggles applies. It also has
a whimsicality about it which to me suggests
house-elves. I'm imagining Dumbledore telling
a Hogwarts elf, "See to it that the letter is
delivered and Harry receives it", with some
additional advice about how Muggles address letters
to a particular location. (I don't remember,
but you seem to indicate that with wizard post,
you just tell the person's name to an owl.)
And so, what happens, happens.