John R. Lott points out in “Why Do Most Countries Ban Mail-In Ballots?: They Have Seen Massive Vote Fraud Problems“:
Thirty-seven states have so far changed their mail-in voting procedures this year in response to the Coronavirus. Despite frequent claims that President Trump’s warning about vote fraud/voting buying with mail-in ballots is “baselessly” or “without evidence” about mail-in vote fraud, there are numerous examples of vote fraud and vote buying with mail-in ballots in the United States and across the world. Indeed, concerns over vote fraud and vote buying with mail-in ballots causes the vast majority of countries to ban mail-in voting unless the citizen is living abroad.
There are fraud problems with mail-in absentee ballots but the problems with universal mail-in ballots are much more significant. Still most countries ban even absentee ballots for people living in their countries.
Most developed countries ban absentee ballots unless the citizen is living abroad or require Photo-IDs to obtain those ballots. Even higher percentages of European Union or other European countries ban absentee for in country voters. In addition, some countries that allow voting by mail for citizens living the country don’t allow it for everyone. For example, Japan and Poland have limited mail-in voting to those who have special certificates verifying that they are disabled.
France has made an exception this year to the ban on absentee ballots to those who are sick or at particular risk during the Coronavirus pandemic. Poland and two cities in Russia have adopted mail-in ballots for elections this year only, but most countries haven’t changed their regulations.
France banned absentee voting in 1975 because of massive fraud in Corsica, where postal ballots were stolen or bought and voters cast multiple votes. Mail-in ballots were used to cast the votes of dead people. Examples for other countries are provided.
You can download the full paper here.