Communications by mailbox

I have a large wall of mailboxes for deal I’m closing. I assume opening entire thing at once and dropping communications is still a huge no no these days correct? Does anyone do this to avoid walking the park or using mail?

I was told when one of my apartment buildings went from a mail slot in each unit’s door to a cluster of mail boxes, that it is illegal for anyone to put anything in the mail boxes except for the post office.