source SENDER authentication ? (as opposed to SPF HOST authentication)

[Michael Grey]

Are there any SA methods that allow verification of the ‘sender’ of an email ?   I am aware of SPF which can confirm that a host at ip address x.x.x.x is authorized to send mail as from domain “A”, but how about a means to confirm that ‘[EMAIL PROTECTED]’ actually is a real user before accepting mail from him ?   Thanks     Michael Grey