Relative newbie here, I came to this thread from a search on the Primal client for "outbox", very helpful thread for getting my head around what outbox is and is not. (Their search is fast but you have to wade through a lot of mastodon stuff sometimes.)

Just to make sure I’m getting it, if I’m replying to your note but you don’t follow me, and we have no relays in common, then you won’t see my reply? But if Person X replies to my reply, and you happen to be following Person X, then outbox should direct your client to fetch my note along with Person X's reply? That is to say they should both show up for you roughly at once, as long as the relevant relays are up?