Vote-by-mail is a secure, effective, and practical voting method, and is virtually immune to the sorts of systemic fraud that plague electronic voting.
Wrong. It's vulnerable to systemic fraud in the counting. If you infiltrate the post office or the election office you can easily alter the results in volume.
When you have a polling location you can verify the box is empty, observe people placing votes into it, and observe the counting. You observe every step in the process to get your poll's final tally, and when the results are posted with a breakdown by polling location you can verify that it was added correctly to the total. All you need is a few trusted people per polling location and you can trust the results.
But in vote-by-mail the only part you observe is casting your own vote. You can't say that a 'household tyrant' didn't vote for others -or- that systemic fraud didn't occur in the post or in the tally. It's better than internet voting could ever be since the unobservable parts (post office, elections office) are harder to corrupt and get away with it, but it's still unacceptable for running fair elections.