The foaf:mbox property is a relationship between the owner of a mailbox and a mailbox. These are typically identified using the mailto: URI scheme (see RFC 2368).

Note that there are many mailboxes (eg. shared ones) which are not the foaf:mbox of anyone. Furthermore, a person can have multiple foaf:mbox properties.

In FOAF, we often see foaf:mbox used as an indirect way of identifying its owner. This works even if the mailbox is itself out of service (eg. 10 years old), since the property is defined in terms of its primary owner, and doesn't require the mailbox to actually be being used for anything.

Many people are wary of sharing information about their mailbox addresses in public. To address such concerns whilst continuing the FOAF convention of indirectly identifying people by referring to widely known properties, FOAF also provides the foaf:mbox_sha1sum mechanism, which is a relationship between a person and the value you get from passing a mailbox URI to the SHA1 mathematical function.