Need mail 'how to'

What I'm asking for likely exists and I've missed it somehow. Please point that out to me.

In Spite, everything works how I expect it. I receive mail, I go to either my house mailbox or the bank mailbox and retrieve my mail.

Everywhere else I've been, it's confusing. Every other town bank I've visited (granted only like 3) doesn't have a bank mailbox. I can't access my mail from someone else's box. I've had a total stranger in Owl's Head ask how they access their mail, and I've told them 'Typically the bank, although I'm not sure where a mailbox is in Owl's Head.'

So...point me to the 'how mail works' place for myself and others. Thanks!