Graham Leggett writes:

Because that's how it works.

It's quiet feasible to print out a paper copy of each incoming mail, and put the paper copy of it into a physical mailbox. Just because something is feasible doesn't mean that's the right way to do it.

Well, I don't know what exactly is advertized, and where, but there isn't anything like that in the documentation.

Crafting an error message for each possible outcome of a comparatively complicated procedure is not very productive. There are literally hundreds of things that can possibly go wrong, and trying to chase down every possible point of breakage is like trying to herd a flock of cats. The rule of thumb that's being followed is that to report only foreseeable failures. It is reasonable to believe that people are going to read the documentation and the instructions before carrying on. If people don't bother reading the documentation, they'll just end up wasting a whole bunch of time.