A mailbox receives mail. It is a conceptual entity which does not necessarily pertain to file storage. For example, some sites may choose to print mail on their line printer and deliver the output to the addressee's desk.
A mailbox specification comprises a person, system or process name reference, a domain-dependent string, and a name-domain reference. The name reference is optional and is usually used to indicate the human name of a recipient. The name-domain reference specifies a sequence of sub-domains. The domain-dependent string is uninterpreted, except by the final sub-domain; the rest of the mail service merely transmits it as a literal string.