yes, relays are like POP in the sense that they cache messages... teasing out the differences from simple cache to authoritative archive is a separate matter, i'm working on this in my every day, can't stop thinking about it of course - we are turning an altcoin system into the archive... my boss doesn't quite fully get it either, he was talking up the idea of relays broadcasting and i was sitting there quietly thinking to myself (dude, bro, we are selling them the idea of their system being the archive)