There is a reason that most countries don’t allow mail-in voting. Here are a few countries that have done away with it.
Norway & Demark — no absentee voting, do allow in person early voting.
Finland has recently introduced mail-in voting but only for Fins living abroad.
France had absentee voting until around 1970, but abolished it b/c of vote-rigging.
Over three elections, a election judge in Philadelphia has pleaded guilty to standing in an election booth and voting as fast as he could over and over again. Domenick J. DeMuro faces up to 15 years in federal prison for this fraud. From the Philadelphia Inquirer:
A former judge of elections and Democratic committeeperson from South Philadelphia pleaded guilty to accepting $2,500 in bribes to inflate the vote totals for three Democratic candidates for Common Pleas Court judge between 2014 and 2016, U.S.