Proponents of mail voting point out that there is no basis to suggest that mail-in voting is not safe from voter fraud. In fact, while the prevalence of mail-in voting will increase in the 2020 election due to COVID-19, mail-in voting has been a long-standing practice within the U.S for decades. And for the decades that this practice has occurred, it has proven to be very secure with only extremely rare cases of attempted fraud at a rate of 0.000006%. 
There is and always has been a system put in place to prevent voter fraud from domestic voters and foreign threats. These anti-fraud protections include requiring people who request ballots to be registered voters, mailing ballots to the official address listed on voter registration rolls and having election authorities thoroughly review mail-in ballots to verify names, addresses, and signatures.
Mail-in voting is safe from voter fraud committed by foreign powers. There is no way for foreign governments to produce and replicate mail-in ballots with any or all of the components that election officials will be checking and verifying on election day. 
Mail-in voting may be safer from voter fraud than in-person voting because people who vote in person don’t always update their address when they move and may be voting at the wrong polling place, whereas mail-in elections constantly update voter addresses to ensure they receive the right ballot. 
There is no basis for the argument and no evidence that mail-in voting leads to voter fraud.