How is anything supposed to extend POP3 then? No Way I guess?
RFC 2449: POP3 Extension Mechanism.
One that occurs to me is to have a last ditch SPAM hook of some kind to plumb in a process to evaluate "grade" messages.
This is either an SMTP mechanism or a local storage mechanism. I don't see where in the POP download process you would put this.
Maybe even authenticate them; (or apply site crypto. Not very strong really).
POP already has authentication (RFC 1734) and TLS (RFC 2595), but I don't think that's what you're talking about. I don't see how crypto or authentication apply to spam in the context of POP here.
pr -- Pete Resnick <mailto:presnick@qualcomm.com> QUALCOMM Incorporated