"This document describes the functional specification and definition of the ENUM Naming Authority Pointer (NAPTR) record for IFax service. IFax is ""facsimile using Internet mail"". For this use@ the Domain Name System (DNS) returns the email address of the referenced IFax system. This mechanism allows email-based fax communication to use telephone numbers instead of requiring the sender to already know the recipient email address."
RFC 4143-2005 history
2005RFC 4143-2005 Facsimile Using Internet Mail (IFAX) Service of ENUM