BOSTON (AP) — Brace yourself for what’s expected to be the first U.S. presidential election conducted mostly by mail. It could be messy, with as many as three in four Americans expected to embrace the option. But running a vote-by-mail election is surprisingly complicated, and there’s a lot of room for things to go wrong.

Validating and counting a deluge of posted ballots in an open and accountable way presents a major logistical challenge, one that only about a half dozen states are fully prepared for. Chaotic events during this year’s primaries did not instill confidence.