[henrikschroder]

Generally, by sealed envelopes, and by having groups of people inspect mail votes at counting time to ensure the envelopes haven't been tampered with. There's also usually a paper trail from the post office that receives the votes so you can't just show up with a couple of thousand "mail votes" and send them in.

It is obviously less secure than voting in person, but it's good enough, and your in-person vote supersedes your mail-in vote.