[hifigi]

In Washington state (King County), the mail-in ballots are placed inside a privacy "sleeve" that contains a signed statement on the outside that you autograph. If your signature matches the signature of your voter registration, the ballot is removed from the privacy sleeve and put into an anonymous pile. It is then processed by a scantron machine. You only get a confirmation that your vote was processed into the system and counted.

[function_seven]

But none of that prevents a Alice from forcing Bob to mark his ballot in the prescribed way, with Alice then taking it to the mailbox herself.

Bob may choose to deliberately change his signature so the ballot is at least invalidated, but that still takes a vote away from his preferred candidate, and would rely on Alice not knowing what Bob's true signature looks like.