Answer: No. Contrary to Trump's tweet that "Mail-In Voting is an easy way for foreign countries to enter the race," it would be one of the most difficult ways for a foreign adversary to meddle in a U.S. election.
Swaying a federal election using absentee ballots would mean paying thousands of U.S. citizens, carefully selected in key cities in battleground states, who are willing to conspire with a foreign government and risk detection and prosecution.
Far easier and cheaper would be a social media campaign seeking to discourage certain groups of people from voting, something the FBI has already warned about. Or launching a sophisticated cyberattack on voter registration data that would eliminate certain voters from the rolls, causing havoc at polling places or election offices as officials look to count ballots from people who are "missing" from their voter databases.
Last month Attorney General Bill Barr raised the possibility that a "foreign country could print up tens of thousands of counterfeit ballots." He argued they would be hard to detect, but that's been disputed by election experts.
Absentee ballots are printed on special paper and must be formatted correctly in order to be processed and counted. Ballots are specific to each precinct, often with a long list of local races, and would be easily identified as fraudulent if everything didn't match precisely.
"This is a complete red herring," said Wendy Weiser, director of the democracy program at the Brennan Center for Justice. "It's completely not plausible, and not something that security and election experts are actually worried about."