Here are 10 examples of it. 2 of them are not mail in voter fraud but plain ol voter fraud
https://www.nytimes.com/2020/07/09/us/politics/west-Virginia-election-fraud.html
https://www.justice.gov/usao-ndwv/pr/pendleton-county-mail-carrier-charged-attempted-election-fraud
https://apnews.com/bc8cd3b0a901480136b9af926a93df27
https://news.azpm.org/p/news-articles/2020/6/24/175443-pima-county-man-convicted-of-voter-fraud/