While doing some research, I came across a 2008 NPR interview with Rick Hansen, then a professor at Loyola Law School in Los Angeles (now at UC Irvine School of Law). Here is his statement on the vulnerability of absentee-by-mail voting to fraud:
Well, I certainly think voter fraud occurs, but almost all of it occurs through absentee ballots. Because, look, if you want to influence the outcome of an election, you’ve got to make sure that people are voting the way you want them to vote. So what better way than to actually look at their ballot, collect the ballot, and pay them after you see they’ve voted the way you want?