The last line, "in some cases it may be further processed and transmitted by another mail system," is exactly my case -- fetching messages with getmail and passing them to TMDA to possibly send a challenge-response.
Those "some cases..." are simply cosmetic in the RFC, describing a range of gatewayed applications (that's the kind of commentary that older RFCs suffer from) without positing any functional mandate. But that's not your situation, anyway. The messages in your case are "delivered to the destination user," by the widely accepted definition of local mailbox delivery. They only resume/restart mail-like transport when you trigger a download.
That is my situation. RFC 821 doesn't dictate what the final destination means, and RFC 821 is intentionally vague on the type of mail systems that might further process and transmit the message. This "cosmetic" statement helps illustrate the intent of the rule -- recording the envelope sender in the return-path header of the message is important in part because of the unforeseen ways a message may be processed by future systems or protocols, such as POP, even though as you said, "when 821 was written, POP3 was not the expected way of retrieving mail."
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James Thornton _____________________________________________ Internet Consultant, http://jamesthornton.com
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