What is a pillar box?

Hi, I’ve just come across this word: pillar box and it turned out that this is another term for post box in the UK. How popular is pillar box nowadays? Also, do people in North America also say post box or do they prefer mail box? So many boxes here :).

Most Americans will say mailbox, but are as likely to say postman as mailman.

I have no opinion on BrE.