I'm not so sure it can be a letterbox. When registering companies with the Handelsregister in Germany, I think there is a legal requirement to have a physical address. At least from my experience.

In the US, most (all?) states require a local registered agent with a non-PO box in the state. A lot of companies do good business as registered agents for out-of-state and foreign companies, where they file the paperwork and forward mail to the desired recipient.

In Australia, all companies must have a registered address, available on the register of companies (which isn't quite as open as I'd prefer, it isn't nearly as good as the UK's Companies House, for instance). This physical address must then have a plaque showing the registered company name roughly at the door.

In reality, just like in the Paradise Papers leaks etc, an individual accountant's office might have 4000 of these lined up next to their door.

Of course, for the vast majority of businesses, this is not much of a hurdle, and for most consumers it isn't all that helpful either, but end of the day, scammers are gonna scam, whatever the obstacles are. Even those that aren't outright scammers, will be paying accountants to have a plaque on their door to show that this is their "physical office" ...

Hopefully such efforts at least help courts assign some blame when the time comes.

It cannot be a letterbox. You must provide a real address that has capabilities to receive mail.

Luckily for a few euros per month you can rent quite nice addresses and the relative mail forwarding services (e.g., once a day, once a week, etc.)

You are right, that what I meant when I said letterbox.

I should have looked up the definition, before using the word.

Given how easy it is to do e.g. DMCA abuse, or claiming random videos on YouTube, I'd say they haven't solved it. There's even a lovely weird expression for it, fly-by-night companies.