The inbox is one object, seen from three routes

The shape of the worry

It usually arrives as a specific fear. Somebody replied while you were away, you come back on a different route, and the reply is not there. The immediate explanation that suggests itself is that you are looking at the wrong mirror.

That explanation is almost never the right one, and reaching for it makes the real cause harder to find.

Why the inbox cannot be per address

A message store belongs to the account it is attached to. The route you took to look at it is not part of the message and not part of the account. If a message existed on one route and not another, the two would not be routes to one market, and the whole idea of mirrors would collapse.

So the structural answer is clean: one inbox, three ways of looking at it. Nothing about the looking changes what is in it.

The ordinary reasons a message is not where you expected

1. It has not been sent yet. The most common explanation and the least satisfying one.
2. You are looking at a different part of the interface than you think, which is easy on a route you use rarely because the layout is less familiar to your eye.
3. The page you are looking at is one your browser had already stored, which is the subject of your own cache.
4. You are signed in as nobody, because the session did not travel with you, and a signed out view of an inbox is empty for the least interesting reason there is.

The fourth reason deserves emphasis

Arriving on an unfamiliar route and finding an empty looking page is very often a signed out page rather than an empty account. The two look different enough when you are paying attention and identical when you are worried.

Checking whether you are signed in before concluding anything is the single most useful reflex in this whole section.

What a genuine difference would mean

Suppose you checked all of that and one route really did show a conversation the other did not. That is not a story about mirrors being out of step. It is a reason to consider that one of the two things you are looking at is not what you think it is.

The study on the difference that matters is about that case specifically, because it is the one kind of divergence that is worth taking seriously.

Why this matters more than the account question

An account either signs in or it does not, so the reader gets an immediate answer. An inbox can look wrong in a way that is ambiguous, and ambiguity is where people build theories.

The theory that gets built here is usually that messages are per address, and once a reader believes it they start avoiding the other two routes in case something is waiting on them. That converts a set of three into a set of one on the basis of a misunderstanding.

The rule that covers it

If something is missing, check that you are signed in, then reload without letting the browser reuse what it already had, then check again on the route you were using before. Only after all three would the question be interesting, and by then it is not a question about plurality at all.

The habit that resolves nearly all of it

Before concluding anything about a message, look at whether the page thinks you are somebody. That single glance separates the largest cause from every other cause, and it takes no time at all.

It is a dull instruction and it is the entire practical content of this study. A reader who builds it into how they arrive on a route will almost never meet the confusing version of this problem, because the confusing version is mostly a signed out page being read as an empty one.

What this study is not. It is not a study about message contents, encryption or what anybody can read, none of which changes with the number of addresses.