French had massive mail voting fraud in 1975, after which they banned it.
The entire point of elections is proving your government is trustable, you can't trust your government with it because then it's like a self signed certificate, circular trust problem.
Who rigs elections in Russia? The government. Who is most likely to rig elections? The government.
You can't trust government with elections, and just because the government shows you documents doesn't mean anything. Governments fabricate documents all the time. If all you require from corrupt politicians to remain in control is a bunch of mailed documents, you can be sure they'll get those documents mailed. I'm just completely dumbfounded people living within countries with these elections think they are democracies.