] To gain improvement in e-mail authenticity while keeping
] as much SMTP compatibility as possible, a method is
] suggested which doesn't change SMTP at all. The idea is
] to store informations about how to verify who is
] authorized to transmit e-mails through SMTP with a
] particular sender address (either full address or - for
] simplicity - only the domain part of the address) in a
] directory service, which is currently the DNS. To be
] precise, the verification consists of two steps, the
] classical pair of authentication and authorization:

The RMX DNS RR and method for lightweight SMTP sender authorization