Colorado mails out a ballot for everyone, and only ~80% max ever vote in voluntary elections (~90% in Australia with mandatory voting).

That means there's an extra 1/4 of the vote that's sitting out there in trashcans or can be harvested and all you need is some basic information and a signature that's similar enough and it's completely undetectable.

It wouldn't be hard for a crime organization or even on a small scale to fake ballots.

For example you find out who doesn't vote in your neighborhood and steal the ballot from their mailbox. Then you go around collecting signatures for climate change or gun rights and get their signature and copy it. It doesn't have to be a good forensic copy with the same stroke order and shit, just so it looks close.

Election with mail-in ballots are not elections.