Move mail around a mailbox

"Archive everything from this sender" is one sentence to say and a thousand messages to undo by hand. Nothing here is dangerous on its own — a label added is a label that can be removed — but the damage is in the volume and in the selection. The user pictures the eight newsletters they remember; the search matches four years of them, including the two receipts they will need in March.

The selection is where this goes wrong, and it goes wrong quietly. A search returns threads, and a thread is not one message: twenty threads from a chatty mailing list can be three hundred messages. Gmail's estimatedTotal is a guess and not the number you will change. And archiving a thread the user was waiting on does not lose the mail — it removes it from the one place they were going to look, which for them is the same thing until they think to search.

So two mechanisms carry this skill and everything below is about using them properly. The dry run turns the selection into a number before the number matters: it expands the threads, resolves the labels, reports how many messages would change, and touches nothing. The undo is what comes back from an applied change — the exact reverse, pinned to the exact message ids — and it is worth something only if you give it to the user instead of dropping it on the floor.