His (and YOUR) claim:
"“MIT did a study, Losing Votes by Mail, which put the loss of mail-in votes in 2016 at 22 percent. About half the ballots were never received or received too late to vote. And one in two ballots mailed in were challenged by Republicans and not counted.”
The unlinked and unfindable) study is WRONG and nowhere close to reality.
The ACTUAL facts are:
According to the Election Assistance Commission's 2016 presidential election report, "Nationally, by-mail voting constituted 23.7 percent of all votes cast in the 2016 election. Approximately 80.1 percent of absentee ballots that were transmitted to voters were returned and processed, with
1.4 percent of transmitted ballots returned as undeliverable and 2 percent reported as spoiled (e.g., the voter returned the ballot and asked for a replacement).
"Ninety-nine percent of absentee ballots categorized as 'returned and submitted for counting' were ultimately counted in the 2016 election. The most common reasons for rejection in 2016 were missing the deadline, the signature on the ballot not matching the signature on the state's records, and the ballot not having a signature."
https://www.eac.gov/sites/default/files/eac_assets/1/6/2016_EAVS_Comprehensive_Report.pdf