How to validate postcode in address field

Hi

I have added an address field to a user profile type called "Registered Users" and set it to appear in mini registration.

Then I set "Yes" to Requirements > Requires ZIP only in order to capture user's postcode.
However users seem to be able to fill any value they wish in it when they sign up.
I am not sure what is the point of selecting ZIP as a mandatory...

Is it not possible to validate value and allow users to sign up only when they enter value validated as zip?

Could you please shed light on how this field and its requirement settings work?

Many thanks

H