A post office is used to host multiple mailboxes and domains under one area. For example, to provide mail hosting for multiple companies, each company would have a post office. A post office can have multiple domains and mailboxes assigned to it.  A small mail server might only have one post office.  Post offices can have the same name as a domain. It is common for hosting companies to use a domain name as a post office name and to only have one domain within that post office with the same name.