Mail in voting is probably secure, but there are enough concerns where many countries with healthy democracies limit who can vote by mail: https://www.newsweek.com/voting-fraud-real-concern-just-look...

> Here is what we found. Besides the United States, there are 36 member states in the Organization for Economic Cooperation and Development (OECD). Forty-seven percent ban mail-in voting unless the citizen is living abroad, and 30 percent require a photo ID to obtain a mail-in ballot. Fourteen percent of the countries ban mail-in voting even for those living abroad.